What is meant by the term 'modularity' in programming?

Modularity in programming refers to the practice of breaking down complex programs into smaller, manageable parts, known as modules. Each module is designed to encapsulate a specific functionality or a set of related functions, which makes the overall system easier to develop, test, and maintain. By decomposing a program into modules, developers can work on different parts of the code independently, which fosters better organization and reduces the potential for errors. It also allows for code reusability, as modules can often be utilized in different programs without modification.

In contrast, combining multiple programs into one can lead to complexity if not managed properly, which doesn't capture the essence of modularity; creating an extensive codebase without structure is exactly what modularity aims to avoid; and developing a single-layer application does not necessarily relate to modularity since it may not involve any decomposition into distinct functional components. Therefore, the emphasis on breaking down complex programs into manageable modules is why this choice accurately represents the concept of modularity.