What is the phase of software development where end-users validate a system?

The phase of software development where end-users validate a system is acceptance testing. This critical stage occurs after the system has been developed and thoroughly tested internally by the development team. Acceptance testing is primarily focused on ensuring that the software meets the specified requirements and is ready for deployment in the real-world environment.

In this phase, actual users interact with the software to assess its functionality, usability, and reliability based on their expectations and business needs. Their feedback is crucial, as it determines whether the software is suitable for release. The goal is to ensure that the product not only works as intended but also fulfills the users' requirements and provides value.

Other testing phases, such as system integration testing, unit testing, and regression testing, serve different purposes. System integration testing focuses on verifying that different components of the system work together as intended. Unit testing targets individual parts of the code to ensure each functions correctly in isolation. Regression testing checks that new code changes have not adversely affected existing functionalities. While these are all important aspects of the software development life cycle, acceptance testing specifically involves end-user validation, making it the correct choice in this context.