Which of the following was most influential in turning American public opinion against the Vietnam War?A) protests organized by Students for a...

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.