A small cubical aquarium has a depth of 1 foot. In the small

Notice that while the answer options are wide apart, volume of fish is very small as compared to volume of aquariums.

Just do \(24^3 - 12^3 = 12^3(2^3 - 1) = 144*12*7 = 12096\)
now the actual difference will be less than 12096 but around this number. So answer has to be (C).

In case the answer options were close, the last digit of the options would have been different so you could have just figured out the last digit.

In case the answer options were close and multiple options had the same last digit as the answer, then it would not have been a GMAT question at all. GMAT doesn't expect you to do such calculations. Even the one done above is a little extreme.