Subanta
Members of Congress were surprised to learn that legislation that was passed to prevent discrimination on the basis of religious beliefs
had been used by convicted felons demanding special treatment while serving terms of imprisonment. A had been used by convicted felons demanding special treatment while serving terms of imprisonment.
B had been used by convicted felons who demanded while serving terms of imprisonment.
C was used by convicted felons who demanded special treatment while serving out their terms of imprisonment.
D was used by convicted felons demanding special treatment while serving terms of imprisonment.
E was used by felonious convicts who had demanded special treatment while serving terms of imprisonment.
There are two events, one in present(members of congress were surprised) and one in the past(legislation was passed) so there’s no point of using ‘had’ since we aren’t dealing with two past events. This eliminates A, B and E.
Down to C and D, the former has a parallelism error : demanded is not parallel to serving
D doesn’t have this error, hence D.
Hope it helps