In geometry, undefined notions (or undefined terms) are basic concepts that are not formally defined but are understood intuitively. The common examples of undefined notions in geometry include points, lines, and planes.
From your options, the correct answer regarding undefined notions would be:
- A line is an example of an undefined notion.
While angles, line segments, and circles are important geometric concepts, they can typically be defined in terms of undefined terms like points and lines.