C
K
N
U
Answer is Wrong!
Answer is Right!
The missing character is $\boxed{\text{N}}$.
The other three characters, $\text{A}$, $\text{K}$, and $\text{U}$, are all vowels. The missing character, $\text{N}$, is a consonant.
Vowels are the letters of the alphabet that make a sound when you say them. The vowels are $\text{A}$, $\text{E}$, $\text{I}$, $\text{O}$, and $\text{U}$.
Consonants are the letters of the alphabet that do not make a sound when you say them. The consonants are all the other letters of the alphabet.
The missing character, $\text{N}$, is a consonant because it does not make a sound when you say it.