Which best describes 'unit testing' in software development?

Unit testing is a software development practice focused on isolating and testing specific components of an application individually. This approach ensures that each unit of the code performs as expected. Unit tests typically evaluate the smallest parts of an application, such as functions or methods, allowing developers to catch bugs early in the development cycle.

The advantage of unit testing lies in its ability to identify problems at an early stage, which can save time and resources later in the project. By testing components independently, developers can confidently make changes or refactor code, knowing that any issues will be caught by the corresponding tests for those units.

In this context, unit testing differs significantly from other practices such as testing the entire application at once, which is more comprehensive but less focused on individual components. Creating user manuals and documentation, as well as optimizing the application for performance, are separate aspects of the software development lifecycle that do not relate to the purpose of unit testing.