Question

which undefined term is used to define an angle?
ray
point
plane
line

Explanation:

An angle is defined as the union of two non - collinear rays with a common endpoint. A ray is an undefined term in geometry (it is defined in terms of other undefined terms like a point and a line). A point is a location, a plane is a flat surface, and a line is a straight path. But the key undefined term used in the definition of an angle is a ray.

Answer:

ray