To what extent was the February Revolution caused by Russia's involvement in the First World War?
I would say that WWI served as a catalyst that allowed the Russian Revolution to happen somewhat earlier than it might otherwise have. However, I would not say that WWI was the actual cause of the revolution. I would argue that the Russian Revolution was actually caused by the backwardness of the Russian economic and political systems.
The real causes of the Russian Revolution stretch back for decades before the outbreak of WWI. For a very long time, Russia had been much poorer than the major countries of Western Europe. Its people did not enjoy the same sort of standard of living enjoyed by people in places like France and Germany. In addition, Russia had one of the harshest and least open systems of government in all of Europe. In the rest of Europe, serious moves towards democracy were very common. People in the rest of Europe typically had a say in their government and extensive personal rights. This was much less true in Russia. Because Russians were poor and because they were oppressed, they were very unhappy with their government. This unhappiness led to attempted rebellions and assassinations long before WWI.
However, WWI did allow the Russian Revolution to happen. This is true for at least three reasons. First, the war weakened the Russian economy. Because of this, many people were even worse off than usual and were very unhappy. Second, the war weakened the Russian government. It had to use most of its resources to fight the war and was therefore less able to crack down on dissent within the country. Finally, the war caused the German government to arrange for the communist leader Vladimir Lenin (then in exile in Switzerland) to return to Russia. Once there, he helped to foment the rebellion that ended up as the Russian Revolution.
Thus, we can say that Russia’s involvement in WWI allowed the Russian Revolution to happen sooner than it might otherwise have. However, we cannot say that this involvement actually caused the revolution.