The question is asking us to "find the missing number ___:7 = 12:21
First, let's see how similar are 7 and 21 - 21 is 7 multiplied by 3. So the gap will be filled in by something that multiplied by 3 will give us 12. So we have : x*3=12, and also: x=12/4=4, so the answer is
C.4
