What is- zsh: segmentation fault in c programming and how to solve this fault?

It looks like there are several issues in the code.

First in old[] array. The datatype is int but the contents are characters. If you want to use the ASCII values then it is fine.

And another issue is using new as the name of the array. new is a c++ language keyword. Better to change the name.

And for the last, the variable n is not initialized. Probably the compiler can through garbage values to n. I modified the code and looks like working fine.

#include<stdio.h>
int main()
{
    int old[9]={'3','5','7','8','8','6','2','3','5'}; // the ASCII values of charancters are assigned
    int new_num[3][3], r, c, n = 0 ; //changed the name of array and initialized n 

    for (r=0;r<3;r++){
        for (c=0;c<3;c++){
            new_num[r][c]=old[n];
            n++;
        }
    }

    printf("New array is:\n");
    for (r=0;r<3;r++){
        for (c=0;c<3;c++){
            printf("%d ",new_num[r][c]); //added a space in printf() for better visualization
        }
        printf("\n");
    }
}

Your problem is that the n variable is uninitialized. This means it's value is undefined, and using it to index an array is undefined behavior. Undefined behavior means anything can happen. Your program might have worked exactly as you planned.

Or you might have gotten a segfault, which is what actually did happen.

To avoid this, n should have been initialized to 0.

As a sidenote, since n++ returns the value of n, then increments it, you could save yourself a line of code:

new[r][c] = old[n++];

Or you might avoid the need for n at all by using some math. Here I've defined constants for the dimensions of your two-dimensional array. I've also locally scoped r and c to their respective loops as they aren't needed outside of that scope. Lastly, I've used size_t rather than int.

#include<stdio.h>

#define ROWS 3
#define COLS 3

int main(void) {
    int old[] = {'3', '5', '7', '8', '8', '6', '2', '3', '5'};
    int new[ROWS][COLS];

    for (size_t r = 0; r < ROWS; r++) {
        for (size_t c = 0; c < COLS; c++) {
            new[r][c] = old[r * COLS + c];
        }
    }

    printf("New array is:\n");

    for (size_t r = 0; r < ROWS; r++) {
        for (size_t c = 0; c < COLS; c++) {
            printf("%d ", new[r][c]);
        }

        printf("\n");
    }
}