In what situation might a departure clearance specify a void time?

Answer: C — At uncontrolled airports.

A void time in a departure clearance is a specific time by which an aircraft must take off; if it does not take off by that time, the clearance is no longer valid. This practice is commonly utilized at uncontrolled airports, where there is no air traffic control actively monitoring departures. At uncontrolled airports, where pilots must rely on their judgement and adhere to the traffic patterns, a void time helps ensure safety and organization. It provides a clear guideline for pilots to follow, preventing uncertainty and avoiding potential conflicts with other aircraft preparing for departure. In contrast, at controlled airports and under FAA regulations, air traffic control manages departures more closely and does not typically use void times, as they can monitor and direct the flow of air traffic actively. Given these procedural differences, the use of a void time is predominantly relevant in the context of uncontrolled airports, making it the correct response.