The missing character is K.
The sequence is a list of letters that are one letter after the previous letter in the alphabet, except for the letter J, which is skipped.
A. C is the first letter in the alphabet, so the next letter in the sequence should be D. B. G is the third letter in the alphabet, so the next letter in the sequence should be H. C. K is the eleventh letter in the alphabet, so it is the next letter in the sequence after J. D. T is the twentieth letter in the alphabet, so it is not the next letter in the sequence.