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Remembering that ALL the bits need to be reversed, 
not just the '1' bits

the following code compiles, links cleanly
and works properly.

#include <stdio.h>
#include <stdlib.h>

#define INT_BITS (32)

int main()
{
    unsigned int input;
    unsigned int result = 0;
    unsigned int i; // loop counter

    printf("\nEnter the unsigned Number: ");
    if( 1 != scanf("%u", &input) )
    { // scanf failed
        perror( "scanf failed" );
        exit( EXIT_FAILURE );
    }

    // implied else, scanf successful

    for(i=0;i<INT_BITS;i++)
    {
      result  <<= 1;
      result  |= input & 1;
      input >>=1;
    } // end for

    fprintf(stdout, "%u\n", result );

    return(0);
} // end function: main