Actual performance of a task is called

In the context of industrial engineering and production management, the actual performance of a task is referred to as an "event." An event represents a specific occurrence or action related to the task or process being monitored or analyzed. It is a significant happening within the context of the task's execution.

The other options do not accurately describe the actual performance of a task:
Option B: An activity typically refers to a broader set of tasks or actions and may include multiple events.
Option C: A duration refers to the amount of time it takes to complete a task but does not capture the specific performance itself.
Option D: None of these is incorrect as "event" is the appropriate term for the actual performance of a task.

So, the correct answer is Option A: An event, which best represents the actual performance of a task in this context.