Which figure is defined as having an infinite extent in one dimension but is straight?

A figure that is defined as having an infinite extent in one dimension while remaining straight is known as a line. A line can be thought of as the set of points extending endlessly in both directions. This concept is key in geometry, as lines serve as the fundamental building blocks for constructing more complex shapes and figures.

In common geometric terms, a line does not have thickness or width—it is purely one-dimensional, which distinguishes it from other figures like a plane, which extends infinitely in two dimensions. A ray, while straight, has a starting point and extends infinitely in one direction, which does not fit the criteria of having infinite extent in both directions. A line segment, on the other hand, has definite endpoints, making it a finite portion of a line.

Thus, a line is the correct choice because it embodies the characteristics of being straight and extending infinitely in both directions.