Answer: Celia did not use enough parentheses
Explanation:
For the Nested IF function to be recognized, a certain amount of Parenthesis should be used to cater for the parentheses used inside the function.
In this case Celia should have used 3 parentheses at the last parentheses to close off the inside parentheses such that the function looks like,
=IF(A3>750,A3*135%,IF(A3>500,A3*110%,IF(A3<100,A3*90%,A3)))