create menu by switch if we choose 1. factorial 2. odd even 3. prime or not prime 4. exit.

#include<stdio.h>
#include<conio.h>
#include<stdlib.h>
 main()
{
   int fact=1,i,a,no,choice,p=0;
   
   while (1)
   {
       printf("\n1 factorial");
       printf("\n2 odd even");
       printf("\n3 prime number or not");
       printf("\n4 exit");


       printf("\n select a opt ");
       scanf(" %d",&choice);
    switch(choice)
    {
        case 1 :
                 printf("enter any no.");
                 scanf(" %d",&no);
                 for(i=1;i<=no;i++)
                 {
                    fact*=i;
                 }
                 printf("your factorial is %d",fact);
                 break;

        case 2 :
                 printf("enter a no.");
                 scanf("%d",&no);
                 if(no%2==0)
                 printf("it is even no.");
                 else
                 printf("no. is odd");
                 break;

        case 3 :
                 printf("select a no.");
                 scanf("%d",&no);
                 for (a=1;a>=no;a++)
                 {
                    if (no%a==0)
                     {
                         p++;
                     }
                 }
                   if(p==2)
                    {
                       printf("%d is not a prime number",no);
                    }
                   else
                   {
                      printf("%d is a prime number",no); 
                   }
                   break;

        case 4 : exit (0) ;
        default : printf("u choose wrong");
     }
   }


}


output :

1 factorial
2 odd even
3 prime number or not
4 exit
 select a opt 1       
enter any no.7
your factorial is 5040

1 factorial
2 odd even
3 prime number or not 
4 exit
 select a opt 2
enter a no.23
no. is odd

1 factorial
2 odd even
3 prime number or not
4 exit
 select a opt3 
select a no.23
23 is a prime number

1 factorial
2 odd even
3 prime number or not
4 exit
 select a opt 5
u choose wrong

1 factorial
2 odd even
3 prime number or not
4 exit
 select a opt 4