What does a person gain be purchasing an indulgence

They gain temporary pleasure and guilt ;)

The person receiving an indulgence gets release from the temporal punishment associated with unforgiven sins and it is granted by the Catholic Church.

Explanation:

An indulgence is a complete or partial remission of the punishment because of a sin. The person that receives the indulgence has to do an action to receive it like doing a prayer but it doesn't forgive the guilt for the sin which requires to have it absolved through Confession. In the Middle Ages, the purchase of  indulgences became a serious problem and that was criticized by the Protestants.

i would say it is a and b.

hope this