Answer is A
See A=0 B=1 C=2 .......... you will get I = 8 ................
Now you just apply the cipher function character wise ,
BY THE WAY IN THE ABOVE FUNCTION M STANDS FOR 1 character one by one of input
So our first character is I
C( I ) = ( KM+13) MOD 26 =(7*8+13)MOD 26 = 17 and if you see 17 is R value
So just from the first character you can get answer RJCH