Final answer:
When a host computer is de-encapsulating a message at the transport layer of the TCP/IP model, it processes the segment PDU.
Step-by-step explanation:
When a host computer is de-encapsulating a message at the transport layer of the TCP/IP model, it is responsible for processing the segment Protocol Data Unit (PDU).
The segment PDU represents the data unit that is used by the transport layer protocols such as TCP or UDP. It contains the transport layer header and the application data being transmitted.
During the de-encapsulation process, the transport layer retrieves the segment PDU from the received packet and delivers it to the appropriate application layer protocol based on the port number.