Convert all of them to meters.
1 km = 1000 meters
6 * 1000 = 6000
6 km = 6000 m
1 cm = 0.01 m
600 * 0.01 = 6
600 cm = 6 m
1 mm = 0.001 m
6000 * 0.001 = 6 m
So we have:
6000 m, 60 m, 6 m, and 6 m.
So 6 kilometers is the greatest.
If you want to know about geometry, the best thing to know would probably what it is. Geometry is a branch of mathematics that deals with the properties and relations of points, lines, surfaces, solids, and higher dimensional analogs.
Supposing you know about the derivative, notice that
so that when 
, the limit is equal to 
and the answer is A.
