Answer
The best unit of measure to express the area of the top of a desk would be square meters.
Work Step by Step
Clearly, units of square kilometers would be too large to reasonably express the area of the top of a desk. Also the top of a desk would have an area of thousands of square centimeters, so square centimeters are too small. The best unit to express the area of the top of a desk would be square meters. A small desk would have an area of about one square meter while the top of a large desk may be several square meters in area.
