In geometry, undefined notions are fundamental concepts that cannot be formally defined, but are understood intuitively. The most common examples of undefined notions include points, lines, and planes.
From your options:
- An angle is not an undefined notion; it can be defined in terms of rays and rotation.
- A line is indeed an example of an undefined notion.
- A line segment, on the other hand, is defined as a part of a line between two endpoints.
- A circle can also be defined in terms of its radius and center, so it is not an undefined notion.
Thus, the correct response is:
A line is an example of an undefined notion.