task-1;
(a)
#include <stdio.h>
#include <string.h>

int main(){
    char str [101];
    int ch,i=0;

    while((ch=getchar())!='\n'){
        str[i++]=ch;
    }
    str[i]='\0';

    for (int j=i-1;j>=0;j--){
        printf("%c",str[j]);
    }
}

(b)
#include <stdio.h>
#include <string.h>

int main(){
    char str [101];
    int ch,i=0;

    while((ch=getchar())!='\n'){
        str[i++]=ch;
    }
    str[i]='\0';
    int j=i-1;

    char *pointer=&str[0], *p;

    for (p=pointer+j;p>=pointer;p--){
        printf("%c",*(p));
    }
}

(c)
#include <stdio.h>
#include <string.h>

int main(){
    char str [101];
    int ch,i=0;

    while((ch=getchar())!='\n'){
        str[i++]=ch;
    }
    str[i]='\0';
    int j=i-1;

    char *p;

    for (p=str+j;p>=str;p--){
        printf("%c",*(p));
    }
}