Which type of line shows an object with perpendicular extension lines at the corners?

The type of line that shows an object with perpendicular extension lines at the corners is a dimension line. Dimension lines are specifically used in technical drawing and blueprinting to denote the distance between two points or the size of an object. They typically feature arrows at each end, pointing to the extension lines, which indicate where the measurement is taken from and often extend perpendicular from the corners of the object being measured. This clear visual structure facilitates accurate interpretation of the dimensions by those reading the drawing.

In contrast, extension lines serve to connect a dimension line to the object, but they do not themselves denote the measurement. Section lines indicate where a cut has been made through an object to show internal features, and reference lines provide additional guidance but lack the specific measurement functionality of dimension lines. The definition and function of dimension lines thus make them the correct choice in this context.