In which Oracle client credentials' file can we find the port number?

The correct choice is the file that typically contains connection details for Oracle Database, including the port number. The tnsnames.ora file is used to define network service names, which include attributes like the hostname, port number, and service name (or SID) of the Oracle database instance. This file allows users to connect to the database without needing to specify all of these details each time they connect, presenting a more user-friendly approach to database connections.

The listener.ora file also exists in Oracle networking but is primarily concerned with configuring the Oracle Net Listener, which listens for connection requests. While it defines the ports on which the listener runs, it is not the file that clients directly reference for connection details during their connection requests.

The sqlnet.ora file is used mainly for configuring Oracle Net parameter settings rather than specifying connection details like port numbers. It deals with aspects such as authentication and protocol settings.

The truststore.jks file is related to Java Keystore functionality, used primarily for securing Oracle applications through SSL/TLS but does not include network connection details such as the port number.

Thus, the tnsnames.ora file is the appropriate choice as it encompasses the necessary connection information, including the port number for accessing the Oracle database.