Answer
The best unit of measure to express the area of a national park would be square kilometers.
Work Step by Step
Since a national park has quite a large land area, units of square centimeters and units of square meters would be too small to reasonably express the area of a national park. The best unit of measure to express the area of a national park would be square kilometers. A national park can be several kilometers long and several kilometers wide, so square kilometers would be the best choice to express the area of a national park.