The correct answer is D. process id.
A process ID (PID) is a unique number assigned by the operating system to identify a process. It is used to reference the process in system calls and to keep track of the process’s state.
A host ID is a unique number assigned to a computer by the network. It is used to identify the computer on the network.
A host name is a human-readable name that is used to identify a computer on the network. It is not unique and can be changed.
An identifier is a general term for a unique name or number that is used to identify something. It can be used to identify a process, a file, a user, or anything else that needs to be uniquely identified.