Step-by-step explanation
Firstly, let us note that the height of a giraffe is a "finite" idea in the following sense: you put the zero in the bottom of the animal and then extend the rule to the highest part of the animal (this highest part is the limit, when you stop the measure).
Recall that ray, plane, and line are "infinite" geometric notions. A ray is a "line starting at a point", which extends to infinite in one direction. A line extends to infinite in two directions; and finally, the plane extends to infinite in all directions.
Answer
The geometric term that best describes the height of a giraffe (and any animal) is segment.