Answer: The correct answer for the blank is-
commensalism.
Commensalism is a long term biological interaction between two organisms belonging to different species where one organism gets benefit and the other organism neither gets benefit nor it is harmed.
In this question, Barnacles living on whales get benefit as they get their food while moving around on Whales. In this interaction, the whales are unaffected.
Thus, this is an example of commensalism.
