The reason if you divide a number to its self or to something, then it would have to be an equal number and if its not an equal number then you will run out or there would be some left out.

I think you are talking about a dividing numbers like 5/2 and getting a remainder of 1? If this is true, the reason for this is not every number will go into every number. There will be a remainder smaller than the number you are diving by

What mathematical statement is missing ? ? the answer is b