Although all of the people mentioned in these options were famous in the 1970s, the only ones who were reporters involved in the Watergate scandal were Woodward and Bernstein.  For this reason, A is the only possible answer.

Answers B and C are meant to confuse you because the people mentioned in those answers were also involved in the Watergate scandal.  However, all of those people were part of the Nixon Administration, not part of the effort to reveal Watergate.  Frank Wills was also involved in the Watergate scandal since he was the security guard who discovered the original burglary.  However, he was not a reporter.

So, the only possible answer is A.