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In a network schedule, the fundamental unit of work is an activity. Activities represent the tasks or work elements that need to be performed to complete a project. Each activity has specific durations, resources, and dependencies, which are essential for planning and scheduling the project effectively.

Understanding activities is crucial for project management because they form the basis for establishing timelines, resource allocation, and the critical path, which determines the overall duration of the project. Activities are detailed enough to allow project managers to estimate effort, assign resources, and track progress throughout the project lifecycle.

The other options represent components of a network diagram but do not serve as fundamental units of work. An arrow typically illustrates the relationship or dependency between activities, while a node can represent the start or end points of those activities. Events mark significant points in a project's timeline but do not encompass the work itself as activities do. Consequently, activities are integral to creating an effective and actionable project schedule.