Mutualism is a symbiotic relationship in which both organisms benefit.
In A, the Oxpecker birds benefit since they get food, and the zebras benefit since they get ticks removed from them. This is mutualism.
In B, this is parasitism since the tapeworm is benefiting from getting nutrients but the rat is being harmed from having its nutrients stolen from it.
C is an example of a predator-prey relationship since the polar bear is eating the seal.
D is an example of commensalism since the birds are getting food from the cattle stirring through the grass but the cows don't care about the birds taking the insects.
So the answer is A.