Correct answer: C) . Measure the total energy of the aquarium before feeding and the energy of the food fed to the fish. Then measure the energy in the aquarium after feeding and determine the difference.  

An isolated system is a system in which neither the energy nor the matter can enter or leave the system. As a result of that,  the total mass of the system remains unchanged.

There is a change in the species present, But, the total mass present will not change based on the law of conservation of matter and energy.