In the C programming language, compilation is not a single step. Before the compiler proper translates your source code into object code, a separate program called the C preprocessor scans the file and performs text-level transformations. Every line that begins with the # symbol is an instruction to this preprocessor and is therefore called a preprocessor directive.

The preprocessor runs first and handles tasks such as file inclusion, macro expansion, and conditional compilation, producing an expanded translation unit that is then passed to the actual compiler. Common directives include:

• #include <stdio.h> — textually inserts the contents of a header file, giving access to declarations such as printf.
• #define PI 3.14 — creates a macro; every later occurrence of PI is replaced by 3.14 before compilation.
• #ifdef, #ifndef, #endif — conditional compilation, letting blocks of code be included or excluded based on defined symbols (widely used in header guards).
• #pragma — passes special implementation-specific instructions to the compiler.

The other choices describe different language elements. Comments are written with // or /* ... */ and are ignored entirely rather than acted upon. Control statements such as if, for, and while direct the flow of execution at run time. Function declarations announce a function's name, return type, and parameters to the compiler. None of these begin with #, so preprocessor directives is the correct answer.