Answer:
0
Explanation:
Take 1/5 as an example of this fraction which is is it's simplest form.
The only factor they have in common is zero meaning zero can go into each number- so to speak.
0 can go into 1 zero times
0 can also go into 5 zero times.
If you replace the answer 0 in that statement with the other numbers/answers, you'll find that it doesn't fit.
(Still using 1/5 as an example)
If your answer is one, one can go into one once, but goes into five five times. it is not common.
That is why and how zero is the answer.
Hope this helps. :)