The correct answer is commensalism.
Commensalism is a type of interaction between two species in which one species benefits while the other is neither harmed nor benefited. In the case of barnacles growing on the back of a whale, the barnacles benefit by having a place to attach themselves and grow, while the whale is not harmed by the barnacles.
Mutualism is a type of interaction between two species in which both species benefit. For example, some plants have insects that pollinate them, and the insects benefit from the nectar that the plants produce.
Parasitism is a type of interaction between two species in which one species benefits while the other is harmed. For example, a tapeworm can live inside a human’s intestines and feed off of the human’s food, while the human is harmed by the tapeworm.
Amensalism is a type of interaction between two species in which one species is harmed while the other is neither harmed nor benefited. For example, a plant that produces a toxin that kills other plants is an example of amensalism.
In the case of barnacles growing on the back of a whale, the barnacles benefit by having a place to attach themselves and grow, while the whale is not harmed by the barnacles. Therefore, the interaction between barnacles and whales is an example of commensalism.