Final answer:
The statement that defines time-boxing is 'c) The activity has a fixed timeframe.' Time-boxing strictly limits an activity's duration, unlike a flexible timeline, ensuring efficiency in project completion.
Step-by-step explanation:
The statement that best describes what it means for an activity to be time-boxed is: c) The activity has a fixed timeframe.
When an activity is described as being time-boxed, it means that the activity must be completed within a predetermined, non-negotiable period. This is often used in project management to ensure that tasks are completed efficiently and that the project progresses at a steady pace. Unlike a flexible timeline, time-boxing restricts the duration to a specific allotment and does not allow for the activity to be extended indefinitely.