Which of the following has a definite length ?

The correct option is a line segment.

Let us understand the geometric properties of each option to determine why a line segment has a definite length:

1. Line: A line extends infinitely in both directions. Since it has no starting point and no ending point, its length cannot be measured. Therefore, a line does not have a definite length.
2. Ray: A ray has one fixed starting point (endpoint) and extends infinitely in the other direction. Because it continues without bound in one direction, its length cannot be measured. Thus, a ray does not have a definite length.
3. Plane: A plane is a flat, two-dimensional surface that extends infinitely in all directions. It does not have boundaries or a definite length or width.
4. Line Segment: A line segment is a part of a line that is bounded by two distinct endpoints. Because both ends are fixed, the distance between them can be precisely measured.

Hence, among all the given geometric figures, only a line segment has a definite length.