What property of ARP causes a reply only to the source sending an ARP request?

The property of ARP (Address Resolution Protocol) that causes a reply only to be sent to the source sending an ARP request is:

“The source MAC address appears in the header of the Ethernet frame.”
When a host sends an ARP request to resolve an IP address to a MAC address, it includes its own MAC address in the source MAC address field of the Ethernet frame. The host that receives the ARP request and has the matching IP address in its ARP table responds with an ARP reply, and it puts its own MAC address in the source MAC address field of the reply frame. This allows the requesting host to associate the IP address with the correct MAC address and update its ARP cache. The ARP reply is unicast and sent directly back to the source that made the ARP request.