The correct answer is C. L.
The missing character is the letter that is in the middle of the alphabet. The letters in the alphabet are A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, and Z. The letter L is in the middle of the alphabet.
The other options are incorrect because they are not in the middle of the alphabet. The letter A is the first letter of the alphabet, the letter B is the second letter of the alphabet, the letter C is the third letter of the alphabet, and the letter D is the fourth letter of the alphabet. The letter R is the eighteenth letter of the alphabet.