findloop(struct node *start)
{
 struct node *ptr,*ptr1;
start=ptr;

 while(ptr!=NULL)
{
  ptr1=ptr->next;
  
  while(ptr1!=NULL)
  {
     if(ptr1->link==ptr->link)
     {
        printf("\nLoop found at %d node",ptr->data);
        exit(0);
       }
    ptr1=ptr1->link;
}
ptr=ptr->link;
}
printf("\nNo Loop found in list");
}

BOOL findloop(struct node *start)
{
 struct node *ptr,*ptr1;
 ptr=start;
 ptr1= start->next;
 while(ptr!=NULL && ptr->next!=null && ptr1->next!=null && 
ptr1->next->next!=null)
   {
     if(ptr==ptr1)
          return FALSE;
     ptr=ptr->next;
     ptr1=ptr1->next->next;
   }
  return TRUE;
}

the approach is to mark the nodes starting from the head.and
follow the links.
in the process if you find a node already marked then there
is loop.

void lopp( node **start)
{
    node *temp,*newnode;
    temp=*start;
     while(*temp && !(temp->next==start))
     { temp=temp->next; }
     if(!(*temp)) printf("Non loop");
     else printf("Loop");
}

same logic as 3rd answer..

typedef struct node
{ int data,mark; /*mark is all initialised to 0*/
  node *next;
 }node;

node* findloop(node* start)
{
 node *p=start;
 while(p)
 {
  if(p->mark==0)
    {
       p->mark=1;
       p=p->next;
    }
   else
    {
       return p;
     }
  }
  return p;
}

answer 2 is correct