In the expression 15+n, the sum of number 15 and variable n is given.
Consider option 1,
If Nick took n cookies from the plate consist of 15 cookies, then expression for the remaining cookies is,

Consider option 2,
The expression for total cost of n CD's with each CD cost $15 is,

Consider option 3,
The expression for total number of pages read by Mathew in n days , with 15 pages every day is,

Consider the option 4,
The total number of students with 15 females and n males is,

So last option is correct for the expresion 15+n.