Which phase involves ongoing support and maintenance of the system?

The phase that involves ongoing support and maintenance of the system is Operation and Sustainment. This phase focuses on the actual use of the system in the field after it has been developed and produced. During this time, activities are conducted to ensure that the system remains operationally effective, which includes performing routine maintenance, providing logistics support, and conducting necessary repairs.

Operation and Sustainment is crucial as it ensures that the system can continue to meet performance requirements and support the mission at hand. This phase also covers the management of resources, training for personnel who will operate and maintain the system, and updating or enhancing capabilities as necessary based on user feedback or technological advancements.

In contrast, the other phases listed are more concerned with the development and acquisition processes that lead up to fielding the system rather than its operational maintenance and support once deployed.