In which situation would the appropriate unit of measure be kilometers?

Since you didn't list choices, I can only give you some guidance on the problem.

A kilometer is the length of 1000 meters. And a meter is about as tall as a normal desk height. So a kilometer is a relatively long distance compared to a desk.

It is also about half the length of a mile, or about 2 times around a normal athletic track.

Look for items that are long distance like that.