Prime numbers are the ones that can only be divided by two numbers: themselves and 1.
Let's analyze our options to see which one does not meet this condition:
7--> 7 can only be divided by 7 and 1 --> 7 is a prime number
9 --> 9 can be divided by 9, 1, and 3 --> 9 is not a prime number
3 --> 3 can only be divided by 3 and 1 --> 3 is a prime number
13 --> 13 can only be divided by 13 and 1 --> 13 is a prime number.
Answer: 9