What do "instrument flight rules" (IFR) regulate?

The correct answer identifies that instrument flight rules (IFR) specifically govern the operation of an aircraft when the pilot relies solely on instruments for navigation and control rather than external visual references. IFR is essential for flying in adverse weather conditions or in situations where visibility is limited, such as in clouds or fog. Under IFR, pilots must follow specific regulatory requirements and procedures, which include maintaining communication with air traffic control, filed flight plans, and adherence to altitude and routing instructions. This regulation ensures safety by providing a structured framework for instrument navigation and traffic management in the airspace system.

The other options do not accurately reflect the scope of IFR. For example, the concept of visual flight rules (VFR) pertains to flying under visual conditions. Air traffic rules for commercial pilots encompass a wide range of regulations but are not limited to instrument-based flying. Ground handling procedures refer to processes related to aircraft operations on the ground, which are not covered by IFR.