This one is easier to work out by elimination than by actual math:
1. We know it's a difference, therefore we need to subtract the volume of the balls from the volume of the box.
B is wrong.2. The box is 3dimensional so we must have 3 linear measurements multiplied together in it's volume calculation.
D is wrong.
3. The sphere volume formula uses radius cubed (not diameter).
C is wrong
A is the only one left and must be correct.
To check:
Volume of the box:

Volume of two balls:

Total volume = box - balls:

This matches
A, just as we thought.