What is the similarity between a structure, union and enumeration?

What is the similarity between a structure, union and enumeration?

2) What is the output of this C code?

#include <stdio.hgt;
     void main()
     {
         struct student         {
             int no;
             char name[20];
         };
         struct student s;
         s.no = 8;
         printf("%d", s.no);
     }

3) What is the output of this C code?

#include <stdio.h>
     struct student
     {
         int no = 5;
         char name[20];
     };
     void main()
     {
         struct student s;
         s.no = 8;
         printf("hello");
     }

4) What is the output of this C code?

#include <stdio.h>
     struct p     {
         int k;
         char c;
         float f;
     };
     int p = 10;
     int main()
     {
         struct p x = {1, 97};
         printf("%f %d\n", x.f, p);
     }

5) What is the output of this C code(according to C99 standard)?

#include <stdio.h>
     struct p     {
         int k;
         char c;
         float f;
     };
     int main()
     {
         struct p x = {.c = 97, .k = 1, 3};
         printf("%f \n", x.f);
     }