Which field in a Transmission Control Protocol (TCP) header is chosen from ephemeral ports?

Source Port

Explanation:

When a connection is establish for an application, it assigns a port that is not in use as the source port. Such ports are known as ephemera ports, which are not related to specific protocol and are only allocated for the period of the connection. All data received on this port (until the connection is disconnected) is sent on to that application.