Which type of rule is marked in millimeters?

The correct choice is based on the standard system of measurement used in the rule. A metric rule is specifically designed to measure lengths using the metric system, which is based on units such as millimeters, centimeters, and meters. The millimeter is one of the smallest units in the metric system and is commonly utilized for precision measurements in various applications, including engineering and construction.

In contrast, an imperial rule typically measures lengths using feet and inches, which are part of the United States customary system of measurement. A standard rule can refer to various measurement types, but it does not imply any specific metric measurements. A hybrid rule may use both measurement systems, but it is not solely focused on metric units. Therefore, a metric rule is distinctly marked in millimeters, allowing for accurate and precise metric measurements.