A constantly running system program process is referred to as a Daemon. In computing, a daemon is a type of program that runs continuously in the background rather than being under direct control of a user. These processes handle tasks such as responding to network requests, system monitoring, or other types of background operations essential for system function. Here are explanations for all options for better understanding:

• Daemons: These are processes that run in the background to perform system-level tasks and services spontaneously without direct interaction from users.
• Processes: A process is an instance of a program in execution, which can be an application or a background service, but not all processes are daemons.
• Threads: Threads are the smallest unit of processing that can be performed in an OS, often part of processes, running inside their context.
• Process Control Block: It is a data structure in the OS kernel containing information about a specific process but not a process type.