Help me in solving GSCP14 problem

I’m facing some difficulty while solving this problem, I didn’t understand the problem.

My code

// Update the '_' in the code below

#include <stdio.h>
#include <string.h>

#define MAX_LEN 1001 // maximum length of input string

int main() {
    int t;
    scanf("%d", &t);
    char S[MAX_LEN]; // declare a character array to store the input string
    for (int i = 0; i < t; i++) {
        scanf("%s", S); // read in the input string
        int len = strlen(S); // get the length of the input string
        char X[MAX_LEN * 2]; // declare a character array to store the concatenated string
        int j;
        for (j = 0; j < len; j++) {
            X[_] = S[_]; // copy S into X
        int k;
        for (k = 0; k < len; k++) {
            X[_+_] = S[_]; // concatenate S with itself
        X[j+k] = '\0'; // terminate the string
        printf("%s\n", X); // output the concatenated string
    return 0;

Learning course: C for problem solving - 1
Problem Link: CodeChef: Practical coding for everyone

The problem ask you to duplicate the input string and then print it, for example: if Input is “ab” then your output should be “ab”+“ab” ,i.e. “abab”. The prototype provides with the basic skeleton to solve the problem, you just have to read it, and apply your own tweaks to get the solution