Answer: The correct answer is B. He could add insulation to the aquarium so that it does not exchange energy with its surroundings.
Explanation-
Isolated system is a system in which there is no exchange of energy and mass. As per the question, heat is radiated from the aquarium. In order to make it an isolated system, insulating or diathermic cover is used over it so that it does not exchange energy with its surroundings.