The missing letter is C. When the alphabets are used to represent the missing space, the sequence becomes "A B C D E F G H I J K L M N O P."
I
d
Puzzle Cheater! Answer is D
d
S
It's K!
T
k comes next, and then l.
U (fourth, third, fourth, third, etc).
The next letter in the sequence would be "A." This sequence alternates between the letters R and G, with the addition of the letter A after each occurrence of R. Therefore, following the pattern, the next letter after G would be A.
T