Question

Which undefined terms are needed to define a line segment?

Plane, point
Line, endpoint
Point, line
Segment, point

Answer 1

Three points make a plane.
Two make a line, so you do not need a plane. A line is always in one plane
A line goes on forever, and a line segment is part of a line

Line segments have endpoints as their only defining points; this is how they are different from lines.