Which of the following system preference items allows a user to enable third-party application installations...

In macOS, the "Privacy" settings under "Security & Privacy" are where users can control which applications are allowed to run, including third-party apps. By default, macOS may restrict apps that are not downloaded from the App Store, but in the Privacy settings, users can enable installations from identified developers or even from any source. The other options like "Keychain" manage passwords and certificates, "Accessibility" deals with assistive technologies, and "Spotlight" is the search feature.