Final answer:
In geometry, an undefined term cannot be defined by other terms and is only understood through examples. The undefined term among the options provided is a plane.
Step-by-step explanation:
The student has asked which of the following is an undefined term: (a) Angle (b) Circle (c) Plane (d) Ray. In geometry, an undefined term is a basic concept that is not defined by other terms but is understood through examples and descriptions. In this list, the undefined term is (c) Plane. Unlike other geometric figures that can be described using undefined terms, a plane itself is an undefined term because it only can be explained using examples and comparisons, but it does not have a formal definition.