The undefined term is, A ray extended infinitely in one direction from an endpoint, the correct option is C.
What is an undefined term?
An undefined term is an expression that does not have an interpretation, the point, line, and plane are examples of undefined terms on which the other terms are defined.
The options are:
Option A : The distance along a line segment is the number of units between two points.
The number of units between two points can be defined and interpreted, so it is not an undefined term.
Option B: Perpendicular lines are lines that intersect at a right angle.
This is true, this can be proved and interpreted, so it is not an undefined term.
Option C: A ray extended infinitely in one direction from an endpoint.
When a ray is extended infinitely in a direction, the length of the ray or the position of the other endpoint cannot be determined and so it is an undefined term.
Option D: An angle is a figure formed by the intersection of two rays at a common endpoint.
This completely defines the angle and so it is an undefined term.
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