Under what circumstance may a pilot cancel an IFR flight plan before completing the flight?

Answer: C — Only if instructed by ATC.

A pilot may cancel an IFR flight plan before completing the flight primarily when instructed by Air Traffic Control (ATC). This is because ATC is responsible for ensuring the safe and orderly flow of air traffic. If a pilot is given an instruction from ATC to cancel their IFR flight plan, it is for operational reasons such as changes in traffic, weather conditions, or other safety concerns. In other scenarios, such as when a pilot simply chooses to cancel the flight plan without guidance from ATC, it can lead to confusion regarding their current airspace status and potential safety risks. ATC relies on clear communication and compliance with instructions for managing air traffic, making the pilot's follow-through on ATC instructions vital for maintaining safety and coordination in controlled airspace. While there are circumstances where pilots may operate under Visual Flight Rules (VFR) and possibly cancel an IFR separately, these typically require prior confirmation and are generally less straightforward than direct communication with ATC. In some cases, regulations state that cancellations should follow ATC protocol to ensure proper handling of air traffic.