Which term is used when an aircraft is flying under Visual Flight Rules?

The term used when an aircraft is flying under Visual Flight Rules is VFR, which stands for Visual Flight Rules. This set of regulations allows pilots to operate an aircraft in weather conditions generally clear enough for pilots to see where the aircraft is going. Under VFR, pilots primarily navigate and control their flight based on visual references outside the cockpit, rather than relying solely on instruments.

This is an essential aspect of aviation, as it emphasizes a pilot's ability to distinctly observe and assess their surroundings, ensuring safe operation under favorable weather conditions. The use of VFR is crucial for navigation and maintaining spacing with other aircraft in uncontrolled airspace, where visual awareness is paramount.

The other terms listed refer to different operational regulations; IFR stands for Instrument Flight Rules, which require pilots to fly primarily by reference to instruments, typically in less favorable weather conditions. MFR and YFR are not standard aviation terms related to flight rules, making them inappropriate in the context of this question.