Answer:
An application-layer message is the data sent between two or more applications. This data can be an email, a voice call, an instant message, a downloadable file, or other type of application-based data. Each application-layer message is divided into one or more packets, which are typically sent via the transport layer of the network.
A transport-layer segment is the packet formed by the Transport layer of the network as it takes the application layer data and transmits it to the Network layer. This packet travels across the network and can be fragmented into multiple segments if it needs to cross a link that has a maximum data size it can handle.
A network-layer datagram is the data unit formed by the Network layer of the network. This datagram is sent to or from the source or destination node and contains information about the route it should take through the network. This layer also performs network-level addressing and routing as the datagram is routed through the network.
A link-layer frame is the smallest data unit sent across a single link in the network. The frame is typically made up of a frame header which contains information about the destination node and link-level addressing and the frame payload which contains the data the link should transport. The frame is then enclosed within a frame check sequence which can be used to detect errors in the frame.