The least stable of all building walls is the __ wall.

The least stable of all building walls is the parapet wall. Parapet walls are typically low walls that extend above the roofline of a building, often serving both a decorative purpose and functioning as a safety barrier. However, their structural integrity can be compromised due to their height-to-thickness ratio, making them more vulnerable to collapse under certain conditions, such as high winds or if they are not properly anchored.

When assessing stability, it is important to consider that parapets are usually not designed to support significant loads or resist lateral forces, which can make them less stable compared to walls specifically designed for structural support like shear walls or even masonry and concrete walls. The other types of walls mentioned, such as concrete or masonry, generally have more robust construction methodologies that enhance their stability and resistance to various loads.