The correct answer here is n²-15. You can solve problems like these by breaking down the question.
Once we read it, we can see that it says "number squared." We know that no matter what else happens, that our variable (n) will be squared, so we get "n²."
Now, we can look back at the beginning where we have "15 less than a number." "Less" will almost always indicate subtraction, and since it is "15 less than a number," we know that it becomes "-15" after the "n²."
When we combine the logic above, we can see that this represents n²-15.
