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

Answered

A point and a line.

Further explanation

Ray is part of the line with one endpoint. Ray is an endless straight path in one direction from a starting point, e.g.,

The arrow above the point shows the direction of the longitudinal beam. The length of the ray cannot be calculated.

Undefined terms are basic figure that is not defined in terms of other figures. The undefined terms (or primitive terms) in geometry are a point, line, and plane.

These key terms cannot be mathematically defined using other known words.