Concept: The question tests on the ability to solve basic questions on Mensuration
Solution:
12 inches = 1 foot
18 inches = 18/12 = 3/2 feet
Let the base of the rectangular box = a. Then the volume = a * a * h = 3/2a^2
3/2a^2 = 24
a^2 = 24 * 2/3 = 16
a = 4
Option A