Final answer:
The correct tool to identify which application is using a particular port number is netstat. This versatile networking utility displays active connections, routing tables, and network interface statuses, including the ports being listened to and the associated process identifiers.
Step-by-step explanation:
To identify which application is using a particular port number, tool a. netstat would be the appropriate choice. The command netstat is a versatile networking utility that can be used on both Windows and Unix-like operating systems.
It displays various network-related information such as active connections, routing tables, and the state of network interfaces. Specifically, using netstat with certain flags (e.g., netstat -ano on Windows or netstat -tulnp on Linux) can show the ports that are being listened to along with the corresponding application process identifiers (PIDs).