The correct option is A 10
We wish to find coefficient of x12 in (x3 + x4 + x5 +...)3
= (x3 (1 + x1 + x2 + ...))3
= x9 (1 + x + x2.....)3
= x9(1−x)3
= x9 ∑∞r=0 3−1+rCrXr
= x9 ∑∞r=0 r+2CrXr
Now to make x12 we need put r = 3
So coefficient to x12 is 3+2C3 = 5C3 = 5C2 = 10