The study aims to understand the effect of the presence of corals on the types of organisms, not specially on the size, lifespan or mass. It would be important, then, to understand the different types of populations found in the two areas, because a population is a group of organisms from the same species. Therefore, understanding the populations would mean understanding what are the types of organisms in each area.
So the correct answer is A. by comparing the populations of organisms found in the samples from the two areas