Which of the following is a major component of the receiver assembly?

The major component of the receiver assembly is indeed the ejector. In the context of a firearm like the M9 Service Pistol, the receiver assembly serves as the backbone of the pistol; it houses critical components that facilitate the operation of the firearm. The ejector specifically plays a crucial role in the cycling of the firearm by directing spent casings out of the chamber after a round has been fired. This ensures that the pistol can properly chamber a new round after ejecting the spent cartridge, which is essential for reliable operation.

While the other components listed, such as the barrel, safety catch, and recoil spring, are important parts of the pistol, they do not reside within or are officially classified as part of the receiver assembly. The barrel is essential for directing the bullet, the safety catch provides a mechanism to prevent accidental discharge, and the recoil spring aids in the cycling of the action, but the ejector is unique in its direct involvement with the functioning of the receiver assembly, making it the correct choice in this context.