Option D, Starfish feed on mussels, so when the starfish were removed the mussels no longer had a predator and their
populations grew unchecked.
In any ecological cycle, population growth of one organism is dependent on population of interconnected organisms.
Thus, for any organism A, if the food (or the population of organism on which it feed upon) increases then the population of organism A will increase while on the other hand if the population of predator is increased the population of organism A will decreases.
Here, Starfish is a predator to mussel and thus when the population of starfish is reduced, the population of mussels has increased.
Hence, option D is correct