In a marine ecosystem, starfish prey upon the common mussel. common mussels feed upon plankton. what is most likely to happen if the number of starfish is increased?

The answer is;

The numbers of mussels would reduce

Planktons population would increase.

This is a phenomenon common in food chains where a reduction in predator population increases the prey population and vice versa.  This is because the species population in a food chain/food web are interlinked and source energy from one another directly or indirectly.

it is either c or d and i know for sure it is not a or b