What type of line features a thin unbroken design in blueprints?

The correct choice is the type of line that is used to indicate the limits of particular features on a blueprint. Extension lines are drawn as thin, unbroken lines and serve the purpose of connecting a dimension line to the feature being measured. They help provide clarity on the location and extent of measurement, ensuring that the dimensions are easily understood.

In blueprints, these lines do not carry any specific measurement value themselves but are essential for establishing a visual link between the dimensions indicated and the actual objects or features on the drawing. Their unbroken nature distinguishes them from other line types, which may have different styles and purposes. Understanding the role of extension lines is critical in interpreting technical drawings accurately, as they provide the necessary context for dimensions and geometric features.