What are macros used for in C?

Macros in C are primarily used for code substitution before compilation, making option B the correct answer. This is achieved through the preprocessor, which is a tool that processes source code before compilation. Macros allow developers to define snippets of code or constants that can be replaced throughout the program wherever the macro is invoked, thus promoting code reuse and reducing redundancy.

For example, a macro can be defined as follows:

#define PI 3.14

Every time 'PI' appears in the code, the preprocessor replaces it with '3.14' during compilation. This can simplify code maintenance, as changing the value in the macro definition automatically updates all instances in the code, avoiding the need to manually change each one.

The other options pertain to different features of the C programming language. Creating complex data structures often involves using structures or unions, which are dedicated types in C. Defining new data types is done through typedefs and structures, not macros. Writing inline assembly code utilizes compiler-specific features and is not related to preprocessors or macros directly, as assembly code is usually embedded in specific syntax that does not rely on simple textual substitution like macros do.