A Witness' Refusal to Answer Whether or Not He Is

A witness' refusal to answer whether or not he is a Communist on the ground that his answer would tend to incriminate him is the most positive proof obtainable that the witness is a Communist. - Senator Joseph McCarthy, 1953
In the quote above, Joseph McCarthy seems to ignore constitutional rights granted by the:
A)First Amendment
B)Second Amendment
C)Fourth Amendment
D)Fifth Amendment