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(according to C99 standard)?

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

3) How will you free the allocated memory ?

4) What is the output of this C code?

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

5) What is the output of this C code?

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