The key to this question is figuring out the lengths of the smaller cubes and then figuring out the lengths of the larger cube.

To determine the length of the sides of the smaller cube, we simply take the cube root of 125, or figure out that if x³ = 125, then x = 5. The volume of a cube is x³ because we multiply the base*height*width, and in a cube all three of those dimensions are the same. Thus, the length of the smaller cube is 5.

Now, because we know that the larger cube has 4 of these smaller cubes as a side, we know that the side of the larger cube  is 5*4 or 20. To get the volume of the larger cube, we must multiply 20*20*20 = 8,000.

We could also solve by knowing that there are 4*4*4 = 64 smaller cubes and then multiply 64*125 = 8,000.

Our answer is D.