The correct option is D 12C6+2
Here, coefficient of t24 in {(1+t2)12(1+t12)(1+t24)}
= coefficient of t24 in {(1+t2)12.(1+t12+t24+t36)}
= coefficient of t24 in {(1+t2)12+t12(1+t2)12+t24(1+t2)12}
(neglecting t36(1+t2)12)
= coefficient of t24=(12C12+12C6+12C0)=2+12C6