Final answer:
The best metric unit to measure the height of a classroom door is centimeters, as they are suitable for an object of this size and granularity.
Step-by-step explanation:
When considering the best metric units to use for measuring the height of a classroom door, we must think about the object and the most sensible unit of measurement. Kilometers are used for measuring very long distances such as the distance between towns, and therefore, are not suitable. Liters are a unit of volume, not length, and hence are not applicable. Millimeters are typically used for very small objects, which a door is not. This leaves us with centimeters, which are a practical and common unit of measurement for objects of this size.
The best metric unit to measure the height of a classroom door is centimeters.