Commensalism
Commensalism is a type of interspecific interaction in which one species is benefitted and the other species is neither harmed nor benefitted.
Relationship between Barnacles and Whale
Barnacles growing on the back of the Whale is benefitted as they have a stable place to live, wander and have plenty of food to take.
However, the Whale in return is neither benefited, nor harmed.
Hence, the relationship or the type of population interaction between them is an example of commensalism.
Final answer
(b) Commensalism