The logic follows here is :-
According to the Alphabetic order the series follows here is :
First letter in the series the logic follows here is :
C = 3
C + 3 = F
F + 3 = I
I + 3 = L
L + 3 = O
O + 3 = R
∴Second letter is opposite letter of First letter from the Alphabetic order.
Thus, the correct answer is 'RI'.
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