Final answer:
The source port identifying the application that sent a message on the source computer is contained within the header of a network frame.
Step-by-step explanation:
The part of a network frame that contains the source port identifying the application that sent this message on the source computer is the header of the frame. Network frames are structured in layers, with the header featuring various types of control information, including the source and destination ports used by the Transport layer protocols like TCP (Transmission Control Protocol) or UDP (User Datagram Protocol). These ports help in identifying the sending and receiving applications on the respective host computers.