Questions: Which undefined term is used to define an angle?

The answer is the fourth one (D): ray.

Explanation for each option:

• Line: A line is an undefined term in geometry, often described as a straight one-dimensional figure that extends infinitely in both directions. It is not used to define an angle.

• Plane: A plane is another undefined term in geometry, described as a flat, two-dimensional surface that extends infinitely in all directions. It is not used to define an angle.

• Point: A point is an undefined term in geometry, representing a location in space with no dimensions. While points are used to describe the vertex of an angle, they are not used to define an angle itself.

• Ray: A ray is a part of a line that starts at a point and extends infinitely in one direction. An angle is defined by two rays that share a common endpoint, known as the vertex of the angle. Therefore, a ray is the correct undefined term used to define an angle.

In summary, the undefined term used to define an angle is a ray.