The answer is A. 5FU.
The series is a simple addition of 4 to the previous number, except for 5FU. The correct sequence should be 1CC, 5FU, 9IT, 13LU, 17OR.
1CC + 4 = 5FU 5FU + 4 = 9IT 9IT + 4 = 13LU 13LU + 4 = 17OR
Therefore, 5FU is the odd one out.