write a C programing to modify program to ignore any comments that begin with //

Sure, here's a modified version of a C program that ignores comments beginning with "//":

#include <stdio.h>

int main() {
    int c, last_c = 0;
    while ((c = getchar()) != EOF) {
        if (last_c == '/' && c == '/') {
            // Skip the rest of the line
            while ((c = getchar()) != EOF && c != '\n');
        } else {
            putchar(c);
        }
        last_c = c;
    }
    return 0;
}

If the program encounters two consecutive slashes "//" it will skip the rest of the line, effectively ignoring any comment.