Which measurement unit would typically be used to measure very small lengths?

Millimeters are specifically designed to measure very small lengths, making them the most appropriate choice among the options provided. The millimeter is one-tenth of a centimeter and one-thousandth of a meter, which means it is a metric unit that is small enough to accurately represent lengths that are less than a centimeter. This level of precision is particularly useful in fields like engineering, manufacturing, and science, where detail is crucial.

In contrast, meters and decimeters are larger metric units and may not provide the level of detail needed for measurements that require precision at the small scale. Centimeters are larger than millimeters and are more suitable for tasks that involve measuring slightly larger objects. Therefore, when needing to measure very tiny lengths, the millimeter is the standard choice.