Question

applying undefined terms in definitions
the undefined terms line and plane are needed to precisely define which mathematical term?
line segment
perpendicular lines
parallel lines
ray

Explanation:

A line - segment is part of a line. Parallel and perpendicular lines are defined in terms of lines in a plane. A ray is part of a line. All these can be defined using the concept of a line. But to define parallel lines precisely, we need the concept of a plane along with the concept of a line. In a plane, two lines are parallel if they do not intersect. Without the concept of a plane, we cannot fully define parallel lines in a three - dimensional space.

Answer:

parallel lines