Which abbreviation represents the metric unit for length?

The abbreviation that represents the metric unit for length is "meter." The meter is the fundamental unit of length in the International System of Units (SI) and serves as the basis for many other measurements in the metric system. It can be used for measuring distances both short and long, making it a versatile unit.

While millimeter refers to a smaller measurement than a meter, it is still a derived unit of length rather than the primary metric unit itself. The term "metric" broadly describes the system but does not specify a unit of measurement. Milligram, on the other hand, refers to a unit of mass rather than length. Therefore, focusing on "meter" accurately identifies the base metric unit for length.