Final answer:
The two fundamental forms of concurrent activity in operating systems are processes and threads. Threads are entities within a process that share its resources and can be executed concurrently.
Step-by-step explanation:
Operating systems typically involve two essential forms of concurrent activity: processes and threads. A process, an executable instance of a program, operates independently with its address space, code, data, and system resources. In contrast, threads are entities within a process that can be scheduled for execution. Threads share the same address space and resources of their parent process, facilitating efficient communication and execution within the process. This concurrency model enhances system performance by allowing parallel execution of tasks.
Therefore, the answer to the fill-in-the-blank question is 'threads,' underscoring their role in concurrent and collaborative computing within the context of an operating system.