15
9
17
11
Answer is Right!
Answer is Wrong!
The missing number is $\boxed{11}$.
The numbers in the sequence are all the prime numbers less than or equal to 20. The prime numbers are 2, 3, 5, 7, 11, 13, 17, 19. Therefore, the missing number is 11.
Option A, 15, is not a prime number. It is a composite number, which is a number that has more than two factors.
Option B, 9, is not a prime number. It is a composite number, which is a number that has more than two factors.
Option C, 17, is a prime number. However, it is not the missing number because it is not less than or equal to 20.
Option D, 11, is a prime number and it is less than or equal to 20. Therefore, it is the missing number.