Which device can you use to unlock your Mac computer's screen saver and approve authentication...

You can use your Apple Watch to unlock your Mac computer’s screen saver and approve authentication requests from macOS and other apps, without having to enter a password. This feature is called Auto Unlock and Approve with Apple Watch, and it requires that you have an Apple Watch with watchOS 6 or later, a Mac with macOS Catalina or later, and that both devices are signed in with the same Apple ID using two-factor authentication. To turn on this feature, you need to go to System Preferences > Touch ID & Password > Apple Watch and enable the option next to the name of your watch. Then, whenever you wake your Mac from its idle state or an app requires authentication, you can double-click the side button of your Apple Watch to unlock or approve the task. References:

Unlock your Mac and approve requests with Apple Watch - Apple Support

Authentication Services | Apple Developer Documentation

Approve App Authentication Requests with Your Apple Watch in Catalina