Examining the properties of each number:
11: This is a prime number (only divisible by 1 and itself)
13: This is a prime number (only divisible by 1 and itself)
15: This is a composite number
15=3×5
17: This is a prime number (only divisible by 1 and itself)
The numbers 11, 13, and 17 are all prime numbers.
The number 15 is the only composite number in the group, as it has factors other than 1 and itself.
Therefore, 15 is different from the others.