Answer:
C. Plane
Step-by-step explanation:
One definition of a circle is: "It is the set of all points in a plane that are at a given distance from a fixed point."
The undefined term is the word "plane."
You could define a plane as a flat surface with zero thickness and extends infinitely in all directions, but it isn't in the definition of a circle.
Euclid defined a plane as "a surface that lies evenly with the lines on it" before he defined a circle, so the meaning was understood.
A, B, and D are wrong. They aren't mentioned in the definition.
