What defines a closed 2D geometric figure?

A closed 2D geometric figure is characterized by being completely enclosed, meaning there are no openings, and it can have various shapes and properties. The correct response points out that such a figure must be composed of straight sides, which is a defining feature of polygons. This includes shapes like triangles, squares, and pentagons, all of which are made up of straight edges.

While curves can be part of some 2D figures, they do not define closed shapes in the context of polygons. Consequently, other options that mention specific properties, such as area or the presence of parallel sides, do not universally apply to all closed 2D figures, given that there are closed figures like circles that do not fulfill these criteria. Thus, the ability of a closed figure to consist of straight sides is a foundational characteristic of many common geometric figures in the 2D space.