Answer: Choice D) cm^2
This is the same as saying "square cm". This is a unit of area, and not a unit of volume. Consider a 1 cm by 1 cm square. It's area is 1 square cm. As another example: a rectangle with sides 7 cm by 14 cm has area 14*7 = 98 cm^2.
Much like how 5*5 = 5^2, we apply this idea to the units which is why cm*cm = cm^2.