Which pair of undefined terms is used to define a ray?

The correct answer is: O point and line.

A ray is defined in geometry as a line that begins at a certain point (the endpoint) and extends infinitely in one direction. Therefore, both a point (the starting location) and a line (which represents the path extending from that point) are essential undefined terms used in defining a ray. If you have any more questions or need further assistance, feel free to ask!