Find the missing number?

The answer is $\boxed{3}$.

The missing number is the number that completes the sequence $1, 3, 5, 7, 9, \boxed{11}, 13, 15, 17, 19$. This is an arithmetic sequence with a common difference of $2$. Therefore, each term is $2$ more than the term before it. The first term is $1$, so the missing number is $2$ more than $1$, which is $\boxed{3}$.

Option A is incorrect because $1$ is not the next number in the sequence. Option B is incorrect because $5$ is not the next number in the sequence. Option C is incorrect because $3$ is the missing number. Option D is incorrect because $7$ is not the next number in the sequence.