Which undefined mathematical term can be described as a one-dimensional set of points with no beginning or end?

a. line segment
b. line
c. plane
d. ray

A line segment has two end points.

A line extends infinitely in both directions in one dimension.

A plane is two dimensional

A ray has one end point and extend infinitely in one direction.

So only the line, answer b., is a one-dimensional set of point with no beginning or end.