Which of the following is NOT a supplementary terrain feature?

A gully is indeed considered a primary terrain feature rather than a supplementary one. In land navigation and military operations, primary terrain features include hills, ridges, valleys, and gullies, which are larger and more significant in scale. Supplementary terrain features provide additional detail and help enhance understanding of the terrain but are not as large or prominent. Features such as fills and cuts are examples of supplementary terrain features, as they refer to changes made in the landscape, typically in the context of road construction or earthworks, and do not dominate the overall landscape as gullies do. Therefore, the correct answer accurately identifies gully as not being a supplementary terrain feature.