Final answer:
During copulation, earthworms store exchanged sperm in a chamber called a spermatheca.
Step-by-step explanation:
When earthworms copulate, each worm exchanges sperm, which is then stored in specialized chambers known as a spermatheca. This reproductive behavior is seen in many hermaphroditic organisms such as insects, worms, and mollusks. During the mating process, earthworms lie with their ventral surfaces opposed and secrete a sticky substance to stay attached. After exchanging sperm, they store the received sperm in their spermathecae for later use, when fertilizing their own eggs. This system allows for the timing of fertilization to be optimal for the offspring's survival. Therefore, the correct answer to the question is c. Spermatheca.