Which of the following describes the purpose of unit testing?

Unit testing is a software testing technique that focuses on verifying the functionality of individual components or pieces of code, typically at the function or method level. The primary purpose of unit testing is to ensure that each small unit of the software performs as expected and meets the specified requirements. By isolating these components, developers can detect issues early in the development process, which can lead to higher quality software and reduced costs associated with fixing bugs later on in the system lifecycle.

Unit tests are usually written by developers as part of their coding process and can be run automatically as part of the integration process. This practice helps in maintaining code quality and supports frequent code refactoring, ensuring that changes do not inadvertently break existing functionality.

While the other choices may relate to aspects of software testing or quality assurance, they do not specifically align with the primary focus of unit testing. For example, evaluating overall system performance involves a broader scope than just individual components, and testing external controls pertains to security and compliance rather than the detailed functionality of code units. Similarly, validating user interface designs relates to usability and user experience testing, which is distinct from the purpose of unit testing focused on code correctness.