Which is the longest 2 km 25m 2500 cm or 3000 mm

2km=2 kilometer
kilo=1000 so 2 km=2000m

25m

2500cm
cm=centimeter=1/100 of a meter
so
2500cm=25m

mm=milimeter=1/1000 meter
so
3000mm=3m

so the longets is obviously 2000m or 2km

So,

Let's state everything in terms of meters (m).

The conversion factor between km and m is as follows:
1 km = 1000 m
Therefore, 2 km is 2000 m.

The conversion factor between cm and m is as follows:
100 cm = 1 m
Therefore, 2500 cm is 25 m.

The conversion factor between mm and m is as follows:
1000 mm = 1 m
Therefore, 3000 mm is 3 m.

Here are all the numbers in meters.
2000 m, 25 m, 25 m, 3 m

We can quickly see that 2000 m is the longest distance.  It is 2 km.