Answer is B
how?

Have a look at below picture and see if something clicks??

It will certainly click, if still not understood,

3 P 8 => (8+3) => 11th letter from back

9 G 11 => (9+11) => 20th letter from back

2 U 4 =>(2+4) => 6th letter from back

3 W 1 => (3+1) => 4th letter from back

7 ? 18 => (7+18) => 25th letter from back

123 = 2×23 – 1 = 46 – 1 = 45
233 = 2×33 – 2 = 66 – 2 = 64
321 = 2X21- 3 = 42 – 3 = 39
840 = 2X40 – 8 = 80 -8 = 72
So
111 = 2X11 – 1 = 22 -1 = 21

Answer is 31.
How?
Look at columns, difference between second column number to first column number is same as difference between third column number and second column number, i.e.

c[i+1] – c[i] = c[i+2] – c[i]

lets assume missing number is X, thus
20 – 9 = X – 20
X = 40 -9 = 31