The answer is c. mutualism.
Mutualism is a relationship between two different species in which both species have benefits. In this example, ants benefit because they feed on aphids' wastes while aphids benefit because ants provide them protection.
Commensalism is a relationship between two species in which only one of them has benefit, and the other one is not affected. Parasitism is a relationship between two species in which only one of them has benefit, and the other one is harmed.