Short Answer: J
Explanation:
This is a pattern and the only logical next letter would be j, because the pattern goes in pairs of threes.
If it is done logically:
C is next to B and B is next to A in the alphebet, that is also true in the pattern above.
E is next to F and F is next to G, that is a fact in the pattern above, and the alphabet.
L is next to K and K is next to an unknown letter, following the pattern K is next to two letters in the alphabet, L and K, since L is already used before K, that leaves you with one answer J.