What is a straight one-dimensional figure that goes on infinitely in both directions?

A straight one-dimensional figure that extends infinitely in both directions is defined as a line in geometry. A line is characterized by having no endpoints, meaning it continues endlessly without termination. This infinite nature is what distinguishes it from other similar figures.

In contrast, a ray has a starting point but continues infinitely in one direction, while a line segment has two endpoints, thus is finite in length. An angle bisector is a specific type of line that divides an angle into two equal parts but is still not infinite in both directions. Therefore, the definition of a line as a straight figure that goes on infinitely in both directions is precisely why it is the correct answer.