Which of the following bases is no longer operational?

Walker Air Force Base is the correct choice as it is no longer operational. Located in Roswell, New Mexico, Walker AFB was an active military installation until it was closed in 1967. At its peak, it served as a significant facility for various military operations, including as a primary base for the Strategic Air Command. Following its closure, the site has since been repurposed for civilian use, including the Roswell Industrial Air Center and other developments.

In contrast, Kirtland Air Force Base, Cannon Air Force Base, and White Sands Missile Range are all still operational and play crucial roles in the U.S. military's active defense strategies and aerospace operations. Kirtland serves as a logistics and support base, Cannon is essential for special operations forces training, and White Sands is vital for missile testing and research.