Find the missing character?

The missing character is $\boxed{\text{N}}$.

The sequence is a list of letters, each of which is the next letter in the alphabet after the previous letter. The first letter is $\text{A}$, the second letter is $\text{B}$, and so on. The missing letter is the letter that comes after $\text{M}$ in the alphabet, which is $\text{N}$.

The other options are incorrect because they are not the next letter in the alphabet after $\text{M}$. The letter $\text{C}$ is the third letter in the alphabet, the letter $\text{G}$ is the seventh letter in the alphabet, the letter $\text{U}$ is the twenty-first letter in the alphabet, and the letter $\text{D}$ is the fourth letter in the alphabet.