The correct answer is B. through structural action.
A flexible pavement is a type of road surface that is made up of a layer of asphalt concrete that is supported by a layer of crushed stone. The asphalt concrete layer distributes the weight of the vehicles that travel on the road to the crushed stone layer, which in turn distributes the weight to the subgrade, which is the underlying soil. The subgrade must be able to support the weight of the pavement and the vehicles that travel on it without deforming or cracking.
Option A is incorrect because the wheel load is not distributed directly to the subgrade. The asphalt concrete layer acts as a structural element that distributes the weight of the vehicles to the crushed stone layer.
Option C is incorrect because the wheel load is not distributed through a set of layers to the subgrade. The asphalt concrete layer is a single layer that distributes the weight of the vehicles to the crushed stone layer.
Option D is incorrect because the wheel load is distributed through structural action. The asphalt concrete layer acts as a structural element that distributes the weight of the vehicles to the crushed stone layer.