Hi,
The answer should be letter D, 12 cm3. In fact, since the square pyramid fits perfectly inside the cube, both its height and the base sides will be as long as the cube side. Let’s call this length is “x”. The volume of the cube is equal to x times x times x, so x^3, while the volume of the pyramid is x times x (the area of the base) times x (height) divided by 3, so (x^3)/3. To find the volume of the pyramid all you have to do is (36 cm3)/3 = 12 cm3
Hope this helps! If my answer was not clear enough or you’d like further explanation please let me know. Also, English is not my first language, so I’m sorry for any mistake.