Why are header guards important in C$++$ header files $(.$h$)?$

Question $7$ options:

A$)$ To prevent the header file from being compiled multiple times within a single translation unit.

B$)$ To ensure that the functions declared in the header file are always included in the final executable.

C$)$ To prevent multiple declarations of functions and variables in different source files.

D$)$ To speed up the compilation process by reducing the number of included files.