Final answer:
The Link Layer Discovery Protocol (LLDP) is the Ethernet protocol that verifies if a communication link between two Ethernet devices is established. It is specifically designed for network devices to share information and learn network topology.
Step-by-step explanation:
The Ethernet protocol used to verify that a communication link between two Ethernet devices has been established is the Link Layer Discovery Protocol (LLDP). LLDP is an open and extensible protocol used by devices to advertise, and discover connected devices on a local area network (LAN). This protocol is useful for network devices to learn about the topology of the network by sharing device and port information.
Other protocols listed, such as Link Aggregation Control Protocol (LACP), Address Resolution Protocol (ARP), and Spanning Tree Protocol (STP), serve different purposes. LACP is for bundling several physical network connections into a single logical link for increased bandwidth and redundancy. ARP is used for resolving network layer addresses into link layer addresses, and STP helps in preventing network loops.