Question

the definition of a circle uses the undefined term
arc
line
plane
ray

Explanation:

A circle is defined as the set of all points in a plane that are at a given distance (radius) from a given point (center). Here, the term "plane" is an undefined term in geometry. An arc is a part of a circle (a defined term). A line is a straight one - dimensional figure (but not directly used in the fundamental definition of a circle in the way the plane is). A ray is a part of a line (a defined term).

Answer:

plane