You want to protect your Mac from unwanted contacts from other computers.

This is the correct answer because turning on the firewall is the best way to protect your Mac from unwanted contacts from other computers when you are connected to the internet or a network. A firewall is a software or hardware system that blocks unauthorized access to your Mac by filtering the incoming and outgoing network traffic. The firewall can prevent hackers, malware, or other threats from accessing your Mac or stealing your data. You can also customize the firewall settings to allow or block specific apps and services from communicating through the firewall. To turn on the firewall on your Mac, you need to go to System Settings > Network > Firewall, and click the Turn On Firewall button. You can also click the Firewall Options button to see more settings and options.  References :  Block connections to your Mac with a firewall - Apple Support ,  A Practical Guide to Securing Your Mac | Reviews by Wirecutter