Insert the missing letter.

C -> F; D -> H; G -> N; H .? ; I -> R
A O
B P
C Q
D R
Could you please help?

Assistance needed.

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Convert letters to numbers, A=1, B=2, C=3,...

ABCDE
FGHIJ
KLMNO
PORST
UVWXY
Z

C -> F; D -> H; G -> N; H .? ; I -> R
3 -> 6; 4 -> 8; 7 ->14; 8-> 16 ; 9 -> 18
So it's lette P