Which rule allows flying in both VMC and IMC?

Flying under Instrument Flight Rules (IFR) allows pilots to operate in both Visual Meteorological Conditions (VMC) and Instrument Meteorological Conditions (IMC). IFR is designed primarily for situations where visibility and cloud cover are such that visual references are inadequate for navigation and control, requiring the use of instruments.

When flying under IFR, pilots must adhere to the regulations that govern instrument flight, which include maintaining specific altitudes, routes, and operational procedures. These rules ensure safety when visibility is limited and allow pilots to navigate through cloud layers or adverse weather conditions that would otherwise not permit flying under visual references.

On the other hand, Visual Flight Rules (VFR) are strictly for operations in VMC, where pilots are able to see and avoid obstacles and other aircraft visually. Special VFR (SVFR) permits flights to operate in controlled airspace with visibility below VFR minima but still requires some level of visual reference. Marginal VFR (MVFR) refers to conditions that just meet VFR minimums but may be borderline.

Thus, the IFR approach provides the necessary framework for flying safely in both VMC and IMC, making it the correct choice for this question.