In my mind, there is one reason why 1917 can be seen as the year of the major turning point in World War I. 1917 was the year in which the United States entered the war. While many other very important events happened in 1917, the entry of the US was the one that did the most to impact the eventual outcome of the war.
In answering this question, I would caution you to focus only on things that changed the outcome of the war, not simply on things that were important to history in general. In particular, I am talking about the Russian Revolution. If I were a teacher asking this question, I would not want to see the Russian Revolution in this answer. Let us discuss why I say this.
To me, a turning point is an event that changes the outcome of the war. The Russian Revolution did not do this. The Russian Revolution was indubitably important to the war and to later history. With the revolution, Russia pulled out of the war, allowing Germany to transfer large numbers of soldiers to the Western Front. The Revolution also led to the creation of the Soviet Union, which had a huge impact on the rest of the 20th century. While both of these things mean the Russian Revolution was important, neither makes 1917 a turning point in WWI. The rise of the Soviet Union did nothing to decide the outcome of WWI as that country was not involved in the war. The Russian Revolution did free up German troops and was therefore important to the war, but it is not a turning point because it did not affect who won the war. It helped the Germans, but they still lost the war. This means, to me, that the Russian Revolution was a major event that affected the course of the war (and of later history), but it was not a turning point in the war.
In my view, the only real turning point in the war that happened in 1917 was the entry of the US. This did not have any serious impacts in 1917 as the US did not actually get soldiers to the Western Front in this year. However, once the US entered the war, the outcome was more or less set. By 1917, the war had settled into a stalemate. Even with the influx of troops freed up by the Russian surrender, the Germans were not able to break through the Allied lines in the West. At the same time, the French and British were not able to push the Germans back. With the entry of the US, the balance of power changed. It was clear that the Central Powers would lose the war because a large new army, along with the material resources of the United States, was going to be thrown into the balance against them. Thus, even though American forces did not actually enter into the fighting in 1917, the fact that the US entered the war in that year makes it the turning point of WWI.
Undoubtedly, the German decision to resume unrestricted submarine warfare proved disastrous. The entry of the United States into the conflict provided the physical resources and manpower that helped spur the Allies to victory.
However, 1917 is crucial for other reasons as well. By then, the stalemate on all fronts was decimating morale and the will to fight everywhere. Many firsthand accounts of soldiers who had fought in the tranches show a deep resentment against the decision-makers of the day, be they politicians or ineffective military leaders; they show an ardent desire for the hostilities to end; and they show a profound sense of the futility of war. In spite of this, most fought on. They knew they were going to the slaughter, yet felt that they had no choice.
From 9 to 12 April 1917, the Battle of Vimy Ridge was fought. Although the bulk of the more than 100,000 allied troops involved were Canadian, many British, French and colonial troops fought along with them, under the leadership of Sir Arthur Currie, commander of the Canadian Corps. Vimy Ridge had been under German control since October 1914. It was widely believed to be impregnable, and previous failed attempts supported this view.
However, following months of thorough planning, preparation and rehearsal, the Canadian Corps took most of the ridge on the first day of the battle, and held all of it by the 12th of April. It remained in Allied hands until the end of the war.
This victory provided an enormous morale boost for the Allied forces, especially the Canadians. It proved that with proper planning and use of terrain and troops, a long-standing stalemate could be broken. The victory was costly and casualties were high, as always on the Western Front. However, it was also one of the most significant battles of the war, psychologically, and it gave men who were psychologically, physically and emotionally exhausted a reason to believe that all was not necessarily lost. This victory brought in a moral component to the war effort, which, combined with the entry of the US in the war, created conditions that would lead to an Allied victory and the end of hostilities the following year.
The main reason why 1917 was a critical year in World War I is that Germany made decisions which led to the entrance of the United States into the war on the side of the Allies. With the intercepting and publishing of the Zimmerman Note and with Germany resuming unrestricted submarine warfare, the United States entered the war. The entrance of the United States was a huge event because the war had bogged down. Germany believed it could win the war before the United States could become a factor. However, the United States mobilized faster than expected and got involved faster than expected. If you view World War I as a scale with weights on both side, the scale was pretty evenly balanced before we joined the war. There was no certainty the Allies would win this conflict before we entered. However, once we entered, the scale swung in favor of the Allies. The United States was the deciding factor in World War I. Thus, Germany's fateful decisions which led to the entrance of the United States in World War I are the main reasons why 1917 was a turning point year in the war.