What must a runway surface be like to be used by Part 135 operators for VFR day flights?

The requirement that a runway surface must have adequate strength is foundational for operations involving aircraft, particularly for Part 135 operators, who must adhere to strict safety and operational guidelines. Adequate strength ensures that the runway can support the weight of the aircraft during takeoff, landing, and taxiing without risk of failure, such as structural cracking or deforming. This is crucial for maintaining safety standards, as an inadequately strong runway surface could lead to accidents or damage to the aircraft.

While there may be different types of surfaces that can be acceptable (such as asphalt or concrete), the key takeaway is that regardless of the material, it must possess the necessary strength to handle the specific aircraft being operated, especially in the various environmental conditions that might be encountered during VFR (Visual Flight Rules) day flights.

The other choices might create confusion but do not reflect the essential safety requirements set forth for runway surfaces used by Part 135 operators.