The Voting Rights Act of 1965 passed in Congress by large majorities in part because
The answer to this question is somewhat subjective. It is difficult to know for sure why members of Congress vote the way they do. You should probably look in your book or your notes to find out what answer you are expected to give.
I would argue that B is the best answer, but an argument can be made that C is actually right.
The Voting Rights Act was certainly made possible by white reaction to the violence in Selma. The fact that Southern whites would react so violently to people who simply wanted to be allowed to vote looked very bad to whites in the rest of the country. As the link below says,
A 1965 march to Selma, Alabama, by Dr. MARTIN LUTHER KING JR. and other civil rights supporters to demand voting rights led to police violence and the murder of several marchers. The Selma violence galvanized voting rights supporters in Congress.
However, it is also generally argued that many people supported civil rights and other legislation because they saw it as something that JFK would have wanted. However, I would argue that, by 1965, the Selma violence was much more fresh in people's minds and more of a factor in creating support for this law.