Select the combination of letters that when sequentially placed in the blanks of the given series will complete the series.
CFL, EIK, GLJ, IOI, _ _ _

Let's analyze the pattern for each letter position:
First letter: C, E, G, I, ...

C (3) $+2=$ E (5)

E (5) $+2=$ G (7)

G (7) $+2=$ I (9)

Next letter: I (9) $+2=$ K (11)

Second letter: F, I, L, O, ...

F (6) $+3=$ I (9)

I (9) $+3=$ L (12)

L (12) $+3=$ O (15)

Next letter: O (15) $+3=$ R (18)

Third letter: L, K, J, I, ...

L (12) $-1=$ K (11)

K (11) $-1=$ J (10)

J (10) $-1=$ I (9)

Next letter: I (9) $-1=$ H (8)

Combining them gives KRH.