The only one of these answers that is really plausible is A. The major goal of the protestors was to get the Vietnam War to end.
The majority of the protestors in Chicago in 1968 were focused on ending the war. This clearly means that D and B are wrong. The protestors did not want to give more aid or to support Johnson's policies because those policies were not going to end the war.
Most of the protestors would have been much more likely to support Eugene McCarthy than Humphrey. Humphrey was the "establishment" candidate and was seen as the candidate who would most likely continue Johnson's policies.
For these reasons, A is the best answer.