We cannot figure out C.
First of all, D should be "3 x 7", not "7 x 3".
As for C, I have never encountered the notation with the comma. My guess would be that it could be used to represent the dimensions of an array. In this case, you can make an array with three rows of 7, or even seven rows of 3. So my guess would be 3,7 or 7,3.
Though this problem may occur in a math textbook, it does not make it a mathematical problem. It looks like an exercise in guessing someone's thinking. So don't worry if you can't solve it. Just help the student with real math.