What is a valid return type for a pointer function in C?

Question

The function *getString() is intended to return the address of the local variable a. However, it gives a warning: return from incompatible pointer type [-Wincompatible-pointer-types]. What would be a valid return type for this function?

#include <stdio.h>
char *getString()
{
  char a[] = "Hello World"; 
  return &a; 
}     
int main()
{
  printf("%s", getString());  
  getchar();
  return 0;
}