When the preprocessor encounters a directive, what does it replace it with?

When the preprocessor encounters a directive, it processes commands intended to manipulate the source code before it is compiled. The most common directives include include statements, such as #include, which instruct the preprocessor to replace the directive with the actual contents of the file specified in the directive.

For instance, when using #include <stdio.h>, the preprocessor replaces this directive with the entire content of the stdio.h header file, effectively integrating external functionality like input and output capabilities into the program. This allows programmers to use functions declared in that header file without needing to write them from scratch themselves.

The other options do not accurately represent what occurs during the preprocessor phase. Default code and stored variables do not pertain to the function of directives in the preprocessor, whereas a null value does not apply either, as a directive will either be effectively replaced with code or result in a compilation error if something goes wrong. Thus, understanding the role of directives in the preprocessor is crucial for grasping how C programs are structured and compiled.