What is the main responsibility of a Data Link Layer protocol?

The main responsibility of a Data Link Layer protocol is to transmit frames across a physical link. At this layer of the OSI model, the focus is on encoding, framing, and addressing for data that will be sent over a physical medium. Specifically, the Data Link Layer is responsible for ensuring that data packets are packaged into frames, adding necessary headers that contain information like source and destination addresses, and controlling how these frames are sent and received across a physical network medium. This layer also takes care of error detection and handling, ensuring that the data has been transmitted without errors.

While other options may relate to various functions within the networking space, they do not directly reflect the primary purpose of the Data Link Layer. For example, managing user authentication is more aligned with application or network security layers, facilitating end-to-end communication pertains to higher layers like the Transport Layer, and converting data packets into bits involves physical layer responsibilities rather than those of the Data Link Layer.