The correct answer is A. Port Number.
A port number is a 16-bit number that identifies a specific process to which an internet or other network message is to be forwarded when it arrives at a server. Port numbers are used in conjunction with IP addresses to identify the specific service that a process is listening on. For example, the port number 80 is used for the HTTP protocol, which is the protocol used by web browsers to communicate with web servers.
The other options are incorrect.
- A serial number is a unique number that is assigned to a device or piece of equipment. It is used to identify the device and to track its history.
- A fragment number is a number that is used to identify individual fragments of a data packet. Data packets are often fragmented into smaller pieces so that they can be transmitted more efficiently over a network. The fragment number is used to reassemble the data packet at the destination.
- A domain number is a unique number that is assigned to a domain name. A domain name is a human-readable name that is used to identify a website or other online resource. The domain number is used to locate the website or resource on the internet.