Answer:
C
Explanation:
To find the volume of a rectangular prism, you need to multiply the width by the length by the height (the three given side lengths).
Only C and D do that. Meanwhile A and B just add them, which is not what you're supposed to do.
So, we know it's not A or B.
After finding the volume of each of the prisms, you need to add them to find the combined volume.
D multiplies them, so it's not D.
Therefore it must be C, which correctly multiplies the side lengths and then adds the volumes together at the end.