Which stage focuses on operations, maintenance, and upgrade activities for the base?

The Sustainment Stage is centered around the ongoing operations, maintenance, and upgrade activities necessary for the base. This stage is crucial because it ensures that the system or infrastructure continues to function effectively after its initial deployment. During the Sustainment Stage, activities such as routine maintenance, performance monitoring, and necessary upgrades are performed to enhance capability and extend the life of the system. This allows for a smooth transition from the initial implementation to ensuring long-term operational effectiveness.

In contrast, the other stages mentioned either focus on the initial setup, development, or transition into extended support, which do not emphasize the continuous operational and maintenance aspects that are critical during the Sustainment Stage. Therefore, the choice of Sustainment Stage highlights the importance of maintaining and improving a base's functionality over time.