scanf return three type of value 0, 1, -1
Example
#include <stdio.h>
int main()
{
int i ;
int j=scanf("%d",&i);
printf("%d",j);
}
1 if input enter is integer
-1 if no input enter
0 if input enter is invalid // for above code if we enter input a or b ,aajjs etc not integer
so best answer is option c if no input from keyboard
D is wrong as we can see output can define by above statement of scanf