Which of the following is a WWII German prototype air-to-air missile?

The Ruhrstahl X-4 is recognized as a German prototype air-to-air missile developed during World War II. It was designed to be launched from aircraft and aimed at enemy bombers. Its development represented a significant step in missile technology, utilizing guidance systems that would have made it more effective against high-speed targets compared to contemporary guns.

In contrast, while the Rheinmetall G-5 was a project associated with anti-tank missiles, it did not focus on aerial combat. The V-2 Rocket, commissioned during WWII, was a long-range guided ballistic missile primarily used against ground targets rather than in air-to-air engagement. Lastly, the Messerschmitt MS-406 is a French aircraft and not a missile. Thus, the Ruhrstahl X-4 stands out as the only correct choice, emphasizing the technological advancements in missile systems during the wartime period.