Which measurement is the greatest between 6 km, 60 m, 600 cm, and 6,000 mm?

This is a metric prefix problem. Some of the metric prefixes are:

In this case, you have:

#=>6"km" = 6 xx10^3 "m" = 6000 "m"#
#=>60 "m"#
#=>600 "cm" = 600 xx 10^(-2) "m" = 6 "m"#
#=>6000 "mm" = 6000 xx 10^(-3) "m " = 6 "m"#

We see that the largest is #6000 "m"#, which corresponds to our #6 "km"#.