What do the extension lines in a blueprint show about the part?

Extension lines in a blueprint are critical components used in technical drawings to indicate the exact location of dimensions. These lines extend from the feature being dimensioned to the dimension line, which contains the measurement itself. They help clarify where the dimension applies on the part, ensuring that fabricators and engineers understand the precise point being referenced, which is essential for accurate manufacturing.

While other elements on a blueprint serve different purposes, like boundaries of the part or assembly instructions, extension lines specifically focus on dimensional reference. This distinction makes option C the most accurate choice regarding what extension lines represent.