Need with what the answer to this question is

The answer is C
C has units of Markers / Markers on the left and Money over an unknown which represents money.

None of the others (D) is incorrect, because we've already chosen C.

A is not because.you have the wrong amount of money divided by the larger number of makers. 

B does not work because the right is markers divided by money. The left had side will have money divided by markers. It won't work. The safest way to  these questions up is to keep the knows on the left and put the unknown on the left with the number it is associated with.
Let's see why the others don't work.