The answer is B.
5 kilometers is a reasonable way to describe the distance.
Describing that distance in such tiny units (such as millimeters) is ridiculous. Yes, 5 Km is the same as 5,000,000 mm, but I can't picture what 5,000,000 of anything really looks like.
And we do use scientific notation for distances. The distance from Earth to the Sun is about 149,597,870 kilometers. Most people would state that as 1.5x10^8 Km, using scientific notation.
