At the Transport layer of the OSI, what is used to find and communicate with a particular application running on a host?

Port number

Explanation:

Port numbers are used in the identification of a particular process for forwarding some form of network message such as the internet when it reaches a server. It is usually a 16-bit integer that is unsigned and found in header that is appended to a message unit. At the OSI transport layer, port numbers are used in searching and communicating with a specific application that is running on a host server.