What is a plane figure?

A plane figure is defined as a two-dimensional shape that has length and width, but no depth. This characteristic is essential in geometry, as it distinguishes plane figures from three-dimensional objects, which have volume. Plane figures include shapes like squares, rectangles, circles, and triangles, all of which can be fully drawn on a flat surface without the need to consider height or depth.

Understanding that a plane figure focuses solely on length and width is crucial in various applications, such as architectural design and engineering, where these two dimensions often play a significant role in planning and schematics. This clarity helps in visualizing and designing structures or systems based on flat representations before considering geometric complexities in three dimensions.

The other options presented do not accurately reflect the definition of a plane figure. For example, a diagram of an aeroplane is a representation of a complex, three-dimensional object, while the notion of a very smooth surface refers more to texture rather than the geometric properties of a figure. Lastly, depicting all parts of an object in various planes suggests a three-dimensional perspective, which again does not align with the definition of a two-dimensional plane figure.