Final answer:
The most appropriate metric unit for measuring the length of a car is meters, as cars are typically 2 to 5 meters in length, making millimeters and centimeters too small and kilometers too large for this purpose.
Step-by-step explanation:
To determine the best metric unit for measuring the length of a car, we must consider the different units of length available within the Metric System, such as the millimeter (mm), centimeter (cm), meter (m), and kilometer (km). As the length of a typical car ranges from about 2 to 5 meters, the most appropriate metric unit to use is meters. Millimeters and centimeters are too small as they're more suitable for objects like jewelry or the dimensions of a small electronic device, respectively. On the other hand, kilometers are used for much longer distances, like the distance between cities, and are too large for measuring a car's length.