Franko, have you tapped on "Show Correct Answer" after hitting Confirm? Please try that next time, then if you are still stuck, post on the discussion forum. Here the choices range from 1 to 20, so we want to list prime numbers up to 40 something (20 + 20) to substitute into x and y: 2 3 5 7 11 13 17 19 23 29 31 37 41 43

Now, try plugging in to eliminate the incorrect choices.
2 -1 =1. So (A) is out
5 - 3 = 2, so (B) is out

We can also try adding the difference to one of the prime numbers, and see if the result is also a prime number:
23 + 20 = 43, so (E) is out
14 +17 = 31, so (D) is also out.

We are left with (C) 13.