This is really not a question that can be answered objectively. Your textbook probably has a sentence or two in it that gives the authors' view of what was most influential. Or perhaps your teacher has said something in class. Either way, you really should check to be sure that there is not a difference of opinion between us and your text and/or teacher.
I would argue that the right answer is C. Pres. Johnson once said that he lost the battle to convince Americans about the war when Walter Cronkite (the anchor of CBS's newscasts) came out and said that he disagreed with the war. This shows how important the media was to Americans' perceptions of the war.
Of the other answers, the most plausible is that the protests helped decrease American support for the war. However, I would argue that most "average" Americans were influenced much more by the media.
Like the above answer stated, there is no perfect answer. In other words, it is a matter of opinion. And the truth is undoubtedly a confluence of reasons. But if I had to choose only one, I would say that reason A would be the most important issue for a few reasons.
Students for a Democratic Society have been active for many years before War and they gained momentum. More importantly, they created a spirit of resistance and revolution. So, when the Kent State Massacre took place, the culture of revolution was already present, even if they did not spearhead the movement.
At this point, a student of journalism, John Filo, took a picture of a fourteen-year-old runaway, Mary Ann Vecchio, screaming over the body of the dead student, Jeffrey Miller, who was shot in the mouth. The photograph won a Pulitzer Prize and became arguably the most famous photograph of the anti-Vietnam war movement.
From this perspective, student movements were central.