C++

Which directive is used to create an “include guard”, which allows a program to be conditionally compiled. This prevents a header file from being included more than once.

		#include
		#guard
		#ifndef
		endif

Answer:

We correct directive is #ifndef to prevents a header file from being included more than once.
It has the following syntax

#ifndef <testmacro>
/* code section 1 */
#else
/* code section 2 */
#endif

So it simply checks whether testmacro is defined or not.If it is defined then it executes code section 1 otherwise code section 2.
So how can we use it?

Parent.h

#ifndef PARENT_H

#define PARENT_H
/*Code of header File*/

#endif

So suppose somebody tries to include this file twice, so in the first inclusion as   PARENT_H is not defined so file will be included with all the code between #define PARENT_H and #endif and PARENT_H will get defined by #define PARENT_H statement. Now if we are including this file again then we will get PARENT_H as already defined so the compiler will skip all the code inside the code between #define PARENT_H and #endif. So the file will be included only once.