What do you call a flat surface that does not curve?

A flat surface that does not curve is referred to as a plane. In geometry, a plane is defined as a two-dimensional surface that extends infinitely in all directions but has no thickness. It is perfectly flat, meaning that any straight line drawn on a plane will lie entirely within that surface, and it has no curvature. This concept is foundational in surveying and other fields involving spatial analysis, as surveys often rely on the idea of a plane to interpret land areas and distances accurately.

In contrast, a slope refers to an inclined surface and does not represent flatness, while a curved surface implies a bending or rounded shape. The term base can describe the bottom or supporting surface of a solid object but lacks the specific connotation of flatness associated with a plane. Hence, the choice of "plane" as the correct answer is aligned with its precise definition in geometric contexts.