In what conditions must pilots operate under VFR?

Pilots must operate under Visual Flight Rules (VFR) in conditions that allow for sufficient visibility to see and avoid obstacles. This requirement is essential because VFR relies on the pilot's ability to navigate and maintain visual reference to the ground and other aircraft.

When visibility is adequate, pilots can effectively scan for obstacles, navigate using visual landmarks, and maintain a safe distance from terrain and other traffic. This is critical for ensuring flight safety and situational awareness, which are cornerstones of VFR operations.

While other options may touch on aspects of VFR, they do not appropriately capture the fundamental requirement that pilots must be able to visually perceive their environment adequately to avoid hazards.