A plurality refers to a situation where the largest group or number is considered, but not necessarily a majority (more than half). In your examples, a plurality would be where a specific number is greater than any other competing number, even if it doesn't constitute a majority of the total.
The examples that represent a plurality are:
A. 370 of 435 members of Congress support a bill.
D. 1,550 of the Democratic Party's 4,765 delegates prefer a candidate.
Both of these provide a greater number than any other competing group, although they do not represent an absolute majority of the total respective groups.
So the selected answers are A and D.