Final Answer:
The metric unit of length among the options provided is cm (centimeter). Thus, the correct answer is a. cm.
Step-by-step explanation:
In the International System of Units (SI), metric units are used to measure various quantities such as length, mass, volume, and more. Among the options given, centimeter (cm) is the unit specifically used to measure length.
The centimeter is a subunit of the base unit for length, the meter (m), in the metric system. It is commonly employed to measure shorter distances, such as the length or width of small objects, the height of a person, or the dimensions of a room. The centimeter is equal to one-hundredth of a meter and provides a convenient scale for measuring lengths that are not best represented in larger units like meters or kilometers.
On the other hand, the options mL (milliliter), mg (milligram), and kg (kilogram) are units of volume, mass, and weight, respectively. Milliliter is a unit used for measuring the volume of liquids, milligram measures mass, and kilogram measures weight or mass on a larger scale than milligrams.
Therefore, among the options provided, only the centimeter (cm) is the metric unit specifically designated for measuring length, making it the correct choice in this context.
Thus, the correct answer is a. cm.