Which length is the longest? $12 \mathrm{~m}$ $12,000 \mathrm{~mm}$ $12,000 \mu \mathrm{m}$ $12,000 \mathrm{~cm}$ $0.0012 \mathrm{~km}$

Step 1 ：Convert all the lengths to the same unit, meters, using the following conversion rates: 1 m = 1 m, 1 mm = 0.001 m, 1 μm = 0.000001 m, 1 cm = 0.01 m, 1 km = 1000 m.

Step 2 ：After conversion, the lengths in meters are: 12 m, 12 m, 0.012 m, 120 m, 1.2 m.

Step 3 ：Compare all the lengths and find the maximum value.

Step 4 ：The longest length is 12,000 cm, which is equivalent to \(\boxed{120 \mathrm{~m}}\).