Find the missing number?

The answer is $\boxed{5}$.

The missing number is the number that completes the sequence $1, 3, 5, 7, 9, \boxed{11}, 13, 15, 17, 19$. This is a sequence of prime numbers, which are numbers that are only divisible by 1 and themselves.

Option A, 6, is not a prime number because it is divisible by 2. Option B, 1, is not a prime number because it is divisible by 1. Option C, 3, is a prime number, but it is not the next number in the sequence. Option D, 5, is the next number in the sequence and it is a prime number.