Answer:
All the individuals of one type of clownfish living in a reef compose: a population.
Step-by-step explanation:
A population can be defined as a group of individuals belonging to the same species that live in one area. Let's break down the information given in the question. The area is the reef. The individuals are the fish living in the reef, and they all belong to a species, clownfish. Thus, this scenario composes a population. Just to make the distinction clearer, if we included individuals of other species (other types of fish, for instance), we would have more populations in the same area and, in this case, we would be describing a community. However, since we are talking only of one species, it is a population.