A C F J _____________?
The answer is O. This is because A = 1, C = 3, F = 6, J = 10.
The series: 1 3 6 10. The difference between each no is increasing by one. i.e, 3-1 = 2; 6-3 = 3; 10-6 = 4; the next difference is 5 and thus the no is 15. The alphabet corresponding to 15 is O.
Sridhar