What is the significance of `#define` in C?

The significance of #define in C lies in its ability to create macros for token replacement. This preprocessor directive allows programmers to define a name for a constant value or expression. When the code is compiled, every occurrence of the defined name in the source code is replaced with the corresponding value or expression.

Using #define enhances code readability and maintainability by providing meaningful names instead of hard-coded values. For example, if a program requires a constant value multiple times, defining it once with #define can easily allow for updates to that value in a single location, avoiding mistakes and ensuring consistency throughout the code base.

The other options relate to different functionalities in C. Defining global variables pertains to variable scope and lifetime, including libraries involves using the #include directive to bring in external code, and managing memory allocation involves functions such as malloc and free, which are unrelated to macros. Thus, understanding the role of #define is essential for effective programming in C.