To force quit an app on a Mac, press the key combination Command + Option + Escape (⌘ + ⌥ + Esc). This opens the "Force Quit Applications" window where you can select the app that is not responding and click "Force Quit." Alternatively, you can click the Apple menu in the top-left corner of the screen and select "Force Quit," then choose the app to quit. You can also force quit an app by right-clicking its icon in the Dock while holding the Option key and selecting "Force Quit" or use the Activity Monitor app to select the unresponsive app and force quit it. If all else fails, you can force quit by opening Terminal and typing the command killall [AppName] to terminate the app forcibly. These methods help deal with apps that freeze or become unresponsive on macOS.