Hi there!
The answer is 3, because 3 can go 3 times into 9.
Explanation:
Have a look at the following three balls (or circles, whatever you can see in it).
ООО
Now have a look at the following nine balls:
ООО ООО ООО
As you can see, the three balls which you've seen first, fit three times into the row of nine balls. Therefore, the answer is 3.
