What does the transport layer use as an addressing method to identify an application running on a host?

Answer

It uses Port Number.

Explanation

Transport layer is an (OSI) that is open system interconnection model which is responsible for end to end communication over a network .

To identify an application running on host the transport layer uses Port Number. A port number is a way of identifying a specific process to which an internet or message are sent when they reach to the  server.