Which of following was considered among the most important events of the counterculture movement?A) The Free Speech Movement // B) Andy Warhol's...

Of the answers that you give us here, the best is C. 
Woodstock is seen as the major event of the 1960s counterculture.  Of the other answers,
only A is in any way plausible.

The Woodstock event is seen
as the most important event of the counterculture because it embodied the things that
the counterculture stood for.  It had rock and roll and drugs and sex.  It had people
out doing what they wanted to do instead of obeying the mores and values of the
"establishment."

The Free Speech Movement in Berkeley was
also important, but it was nowhere near as iconic as Woodstock
was.