System testing is concerned with:

System testing focuses on evaluating the behavior of an entire system or product to ensure it meets the specified requirements and functions as intended. This level of testing is performed after integration testing and is critical as it examines the application in its entirety, considering all components and interactions within the system.

During system testing, the testers validate all aspects of the application, including how the system behaves under various conditions, functionalities, performance, security, and usability, based on the criteria established in the project's scope. This holistic approach helps identify defects that could potentially affect the user experience and operational effectiveness of the system as a whole.

The other options address different aspects of the software testing lifecycle or specific points of responsibility and do not encapsulate the comprehensive nature of system testing. For instance, finding defects in individual software modules pertains more to unit or integration testing rather than a systemic evaluation. The mention of user responsibility does not align with the formal testing processes conducted within a development team. Lastly, testing modifications, migration, or retirement specifically targets transitional phases and is not directly related to the overarching goal of validating the system in its implemented state.