The most common protocol used at OSI Layer 3 is __________.

The most common protocol used at OSI Layer 3 is the Internet Protocol (IP). This protocol is fundamental for facilitating communication across networks by enabling devices to identify and locate each other through addresses. Specifically, IP is responsible for addressing and routing packets of data between devices on a network or across the internet.

At Layer 3 of the OSI model, which is known as the Network Layer, the primary function is to handle packet forwarding including routing through intermediate routers. Other protocols, like Ethernet, operate at Layer 2 (Data Link Layer) and are responsible for local area communication; they do not handle the routing of packets across multiple networks. In contrast, while sockets are a programming abstraction for managing network connections, they do not refer to a specific protocol or function at Layer 3. BitTorrent, a file sharing protocol, operates at higher layers and is not aligned with the functionality of the Network Layer.

Thus, when considering OSI Layer 3's responsibilities, IP stands out as the cornerstone protocol, making it the correct choice.