Code:
#include <stdio.h>
#include <stdlib.h> /*for exit funcion,& strtol*/

int main(void){
int distance_source, gas_amount_pkm, gms_amount_pkm_temp, pollutant_no,long_input;
char *loc; /*for strtol*/

printf("%s\n","1. Carbon Monoxide\n
2. Hydrocarbons\n
3. Nitrogen oxides\n
4. Non methane hydrocarbons");
if((fgets(input,sizeof input,stdin))==NULL){
    printf("error occured getting input\n");
    exit(1);
}
long_input=strtol(input,&loc,0);
is that a start you can use or do you need more help?