The correct option is
D I
By Decoding the given Sequence in the way A→1;B→2;....Z→26
R,M,?,F,D,C→18,13,9,?,4,3
The difference between two successive elements of sequence is decreasing by 1
18−5=13
13−4=9(?)
9−3=6
6−2=4
4−1=3
Hence the missing number is 9 in decoded language i.e I