Mutualism is the event in ecology when two different species engage in a relationship from which both benefit. The benefit can be either in the form of a resource or in the form of a service. Mosses living on tree trunks sap nutrients from trees and are harmful for them, so this fails to show mutualism. Also, the birds do not help the grazing animals in any way thus this relationship also fails the crieterion. Finally, barnacles are obligate symbiotic with the whales; they need to be attached there. While the whale is providing a service, barnacles do not help back so this is wrong too.
The correct answer is b; the bees get nectar while pollinating flowers and the plants get the service that their gametes are spread and they reproduce.