What backdoor pilot episode aired at the end of The Andy Griffith Show's fourth season?

The backdoor pilot at the end of The Andy Griffith Show’s fourth season was Gomer Pyle, U.S.M.C.

The episode follows Gomer as he prepares to enlist in the Marine Corps. Andy accompanies him to the recruiting base, where Gomer meets the hard-driving Sergeant Vince Carter, played by Frank Sutton. The episode introduced the premise and principal comic conflict of the eventual spin-off without requiring a separate pilot broadcast.

Although the pilot was filmed in 1963, it aired on May 18, 1964, as the fourth-season finale. The standalone series premiered on September 25, 1964, with Jim Nabors as Gomer and Sutton returning as Carter. Gomer Pyle, U.S.M.C. ultimately ran for five seasons and 150 episodes.

The other options belong to different parts of the Mayberry franchise. Mayberry R.F.D. followed The Andy Griffith Show after Andy Griffith’s departure, while Return to Mayberry was a 1986 reunion telemovie. The New Andy Griffith Show was a separate later series, not this spin-off pilot.