Find the missing number?

The answer is 11.

The numbers in the sequence are all the prime numbers less than or equal to 16. The prime numbers less than or equal to 16 are 2, 3, 5, 7, 11, 13, and 17. Therefore, the missing number is 11.

Option A, 8, is not a prime number. It is a composite number, which is a number that is divisible by more than 1 and itself.

Option B, 15, is not a prime number. It is a composite number, which is a number that is divisible by more than 1 and itself.

Option C, 5, is a prime number. However, it is not the missing number in the sequence.

Option D, 11, is a prime number and it is the missing number in the sequence.