What does the term 'knoll' refer to in landscape terminology?

The term 'knoll' in landscape terminology refers specifically to a small grassy hill. This definition highlights its distinctiveness as a minor elevation in the terrain, typically characterized by a gentle slope and sometimes covered with vegetation, such as grass. Knolls often contribute to the visual interest of a landscape by providing a slight rise that enhances the overall topography.

In contrast, the other options reflect different landscape features: a flat expanse of land refers to plains or plateaus, a type of tree indicates a plant species rather than a topographic feature, and a large valley denotes a significantly lower area surrounded by higher land. None of these accurately encapsulate the concept of a knoll, which is specifically focused on a small elevation.