Foundation walls are constructed on top of which of these items?

Foundation walls are constructed on top of footings because footings are the structural elements that provide a solid base for the walls. They are typically made of concrete and are designed to distribute the weight of the building's walls and upper structures to the ground below, preventing settling or movement.

Footings are critical in ensuring stability, particularly in varying soil conditions, as they help to anchor the foundation walls securely. They also help in preventing damage that may arise from environmental factors such as freezing and thawing cycles, which can affect the ground beneath the building.

The other options, while related to the foundation and construction process, do not serve as the immediate base for foundation walls. For instance, a slab is a flat, horizontal surface that is often poured at ground level, piers are vertical supports that elevate structures and are used differently, and basement walls are simply vertical structures that are often part of the overall foundation system but do not serve as the base for the walls.