Which of the following is considered a supplementary terrain feature?

A supplementary terrain feature is a type of geographical feature that provides additional detail in understanding the main features of a landscape, particularly in the context of military operations, navigation, and topography.

The term "fill" serves as a suitable answer because it refers to the material used to raise the ground level, typically for construction or landscaping purposes, and is important for understanding both the terrain's usability and its tactical implications. Understanding where fill has been placed can help soldiers navigate and analyze their surroundings, as it can alter natural contours and drainage patterns.

In contrast, features like valleys, mountains, and plateaus are considered primary terrain features as they represent significant and recognizable high or low points in a landscape that form the basis of terrain analysis. While they are vital to understanding the overall topology, a fill is generally regarded as a secondary or supplementary feature because it impacts the terrain but is not one of the main geographical formations.

Thus, recognizing fill as a supplementary terrain feature helps with comprehensive terrain analysis in military contexts.