Pledging to vote can help a candidate all of the following ways EXCEPT:

Pledging to vote for a candidate would be only positive for this candidate: so we have to choose an option that would not be a positive outcome for this candidate.

Such an option is "making the candidate appear weaker" and indeed, pledging to vote for a candidate would not make this candidate appear weaker.  (answer B)

Explanation:

the answer is c.richard m. nixon

your answer would be (a)