Answer will be (B)
Say 1 element from m element set choose 1 element from n element n ways
Now, next element of m " " " 1 " " n element in (n-1) "
" " " " " " " " 1 " " n " " (n-2) "
.......................................................................................................................
" when m=n " " " " " " " 1 " " n " " 1 ways
(As we know max value of m could be n)
So, total no of injective maps from a set of cardinality m to a set of cardinality n is =n.(n-1). (n-2). .....1=n!