9/25 is already as simplified as possible.
This is because 9 and 25 do not have common factors that can go into each other besides the number 1, but if divide each by one, they stay the same.
Common factors of 9: 1, 3, 9
Common factors of 25: 1, 5, 25
None of the factors are the same, except the number 1, but the reason why we can't use that number is at the top. I hope i helped :)