During unit testing, what is primarily tested?

During unit testing, the primary focus is on individual components of the software. This type of testing is conducted to validate that each component or module functions as intended in isolation before it is integrated with other parts of the application. Unit tests typically evaluate specific functions or methods, ensuring they return the expected results for given inputs. This approach helps developers identify and fix bugs at an early stage, thereby improving the overall quality and reliability of the software.

In contrast, the other options focus on broader aspects of the application. Overall application performance, for instance, is assessed at a later stage through integration testing or performance testing, where multiple components are evaluated together. The database connection is sometimes tested as part of integration tests, where database interactions are checked alongside the business logic. Similarly, user interface design is evaluated through user acceptance testing or usability testing, targeting the user's experience with the application rather than individual software components.