Under which condition may a pilot intentionally deviate from ATC clearance or instructions?

A pilot may intentionally deviate from Air Traffic Control (ATC) clearance or instructions primarily in specific situations where safety is a concern. When an amended clearance is obtained, it means that the pilot has communicated with ATC and received new instructions, which allows for a change in the original plan. During emergencies, pilots have the authority to act in a manner that ensures the safety of the aircraft and its occupants, even if it means going against prior instructions. Additionally, collision avoidance is a critical concern; if a pilot perceives an imminent risk of collision with another aircraft, they must take appropriate action to avoid it, even if that requires deviating from ATC instructions. These provisions ensure that pilots can prioritize safety over strict adherence to flight plans or instructions in critical situations.

The other options present scenarios that do not justify deviation from ATC instructions. For instance, personal convenience does not meet safety standards for deviation, and simulating emergencies is not a valid reason to disregard ATC guidance. Similarly, visibility below minimum standards could call for different procedures, but it is not a direct reason for deviating from an ATC clearance without appropriate communication. Thus, option C encompasses the essential conditions where a safe deviation from ATC clearance is warranted.