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

$6 " km"$

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

math.com

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 $6 "km"$ .