A bank of which type of soil will stand best in a vertical wall?

Clay is the correct choice because of its unique properties that make it suitable for forming a stable vertical wall. Clay soils have fine particles that are tightly packed together, which allows them to hold water and maintain a cohesive structure. This cohesion is crucial when constructing a vertical wall, as it helps prevent the wall from collapsing under its own weight or due to external pressures.

Additionally, clay exhibits plasticity; it can be molded and shaped when wet, allowing for the creation of structured forms that can retain their shape when dry. This characteristic of clay enhances its ability to stand as a vertical wall without crumbling or sliding away.

In contrast, sand is generally too loose and lacks cohesion, making it prone to collapsing when not contained. Loam, while beneficial for agricultural purposes due to its balanced texture, doesn't provide the same level of structural stability as clay when required to maintain a vertical form. Silt, while it can hold some moisture, is also less cohesive than clay and is more prone to eroding or shifting under stress. Thus, when constructing vertical walls, clay is the preferred choice due to these advantageous properties.