The correct option is D (15)!×6!
Domain ={1,2,3,4,...,20}
Co-domain ={1,2,3,4,...,20}
For, k={4,8,12,16,20}
f(k)={3,6,9,12,15,18}
Number of ways for mapping f(k) for above k
=6×5×4×3×2=6!
Number of ways to arrange f(k) for remaining k=(20−5)!=15!
∴ Total number of functions =6!×15!