Strings - General Questions

What will be the output of the program?

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

int main(void) {
    char a[20] = "Good ";
    strcat(a, "day");
    printf("%s\n", a);
    return 0;
}

What will be the output of the program?

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

int main(void) {
    char s[] = "world";
    printf("%c\n", s[strlen(s) - 1]);
    return 0;
}

What will be the output of the program?

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

int main(void) {
    char s[] = "racecar";
    int i = 0, j = (int)strlen(s) - 1, ok = 1;
    while (i < j) if (s[i++] != s[j--]) ok = 0;
    printf("%d\n", ok);
    return 0;
}

What will be the output of the program?

#include <stdio.h>

int main(void) {
    char s[] = "abc";
    int i, n = 0;
    for (i = 0; s[i] != '\0'; i++) n++;
    printf("%d\n", n);
    return 0;
}

Which header must be included to use strlen?