How is a flat map best described?

A flat map is best described as a two-dimensional representation of a sphere because it translates the surface of the Earth, which is three-dimensional, onto a flat plane. This process involves various methods of projection, such as the Mercator or Robinson projections, which aim to preserve certain characteristics of the Earth's geography while altering others. Therefore, while a flat map can provide valuable information about distances, directions, and locations, it cannot fully capture the three-dimensional aspects of the Earth's surface.

In contrast, the other options highlight different types of maps or representations. A three-dimensional model of the Earth is not flat and is typically used for educational purposes to visualize geographic features. A folding map used for hiking refers specifically to a type of physical map designed for outdoor navigation, which may not necessarily be flat but is typically detailed and portable. Lastly, a digital map accessible online represents a modern way to view geographic information and includes interactive elements, but it differs from the traditional flat representation. Thus, the most accurate and straightforward description of a flat map is indeed its nature as a two-dimensional representation of a sphere.