Which description best defines a macro in C?

• A. A runtime function that runs during program execution
• B. A type alias created with typedef
• C. A keyword to declare inline functions
• D. A preprocessor directive that defines a token replacement

Answer: (D)

Macros are defined by preprocessing directives that replace tokens in the source code before compilation. This means the preprocessor scans the code and, wherever a defined token appears, it substitutes the replacement text, effectively turning code into a larger, expanded version before the compiler sees it. This lets you create constants or mini code templates that expand in place, such as defining a constant like PI or a function-like macro that computes something on arguments.

Unlike a runtime function, a macro is not a callable unit and it bypasses type checking, getting expanded directly in the text of the program. It also isn’t a type alias created with typedef, which simply gives another name to a type for the compiler. Nor is it an inline function, which is an actual function with type safety and proper scoping that gets compiled and linked like any other function. Macros can be powerful, but they can also introduce issues like lack of type safety and multiple evaluations of arguments, so they require careful use.