Answer:
#ifndef
Explanation:
#ifndef checks whether the given token has been #defined earlier in the file or in an included file.
It should be understood that #ifndef preprocessor commands are used to test if a preprocessor variable has been "defined".
And it should be noted that #ifndef is known as header guards. And its primary purpose in C++ is to prevent C++ header files from being included multiple times.
In conclusion , it should be noted that #ifndef is one of the conditional processing blocks, andit is known as header guards because it used in creation of "include guard".
