In a closed primary
a. only declared party members may vote.
b. all qualified voters may vote.
c. voters must vote a split ticket.
d. voters must complete separate ballots for both parties.

In a closed primary, only declared party members may vote. The advantage of having a closed primary is that it prevents one party from raiding the other primary in the hopes of nominating weaker candidates in the other party.

The answer would be letter A.