Us senator joseph mccarthy was best known for a. leading the call for war in vietnam. b. an overzealous pursuit of suspected communists. c. an overzealous pursuit of suspected fascists. d. voting for the civil rights act.

I agree with the person above - the correct answer is indeed B. an overzealous pursuit of suspected communists.
During the Cold War, which occurred between America and Russia, tensions were high, so it was almost expected that people would be paranoid that there were Russian communist spies in America, which is something McCarthy was convinced was the case.