Which flight rule must be maintained when the weather does not develop as forecasted?

When the weather does not develop as forecasted, maintaining Visual Flight Rules (VFR) is essential for safe navigation. VFR conditions imply that the pilot has the visual references necessary to operate the aircraft safely outside of controlled airspace or to remain clear of obstacles and other traffic. Under VFR, pilots are required to fly with sufficient visibility and distance from clouds, allowing for better situational awareness and the ability to make informed decisions when facing variable weather conditions.

If the weather dips below the minimums necessary to maintain VFR, pilots must either adjust their flight plans or transition to Instrument Flight Rules (IFR), which would require relying on instruments rather than visual cues. However, as the question states, the fundamental requirement when the situation does not unfold as expected is to ensure that VFR conditions are adhered to when feasible. This allows for enhanced flexibility in responding to rapidly changing weather conditions, ensuring safety and compliance with regulations.

The other options, like IFR and SVFR, would not be the first protocols considered unless the weather conditions impose limitations that necessitate instrument navigation or special VFR rules, typically used in controlled airspace under specific circumstances.