Try using a bit of common sense, shall we?
5 mm is less than half the width of a finger.
5 cm is about the length of a finger
5 dm is about the length of your forearm (and hand)
5 m is about three times as long as you are tall.
Now see if you can work out which is the most appropriate.
Meters
length would be about 1dm, but dms are not used. centimetres would be more mainstream to use
The answer depends on what you wish to measure: its length (or width or height), its mass, it fuel efficiency, it maximum speed, its maximum acceleration, its tightest turning circle, its power output, its drag, etc.
A tape measure that measure in feet and inches (or meters and centimeters) would be most appropriate.
Most locks have keys that are small enough to fit into the pocket. As such, a key is about 3 centimeters long.
kilometers
It depends on what you're measuring it for. You could be testing it for the following measurements: Height (using meters would be the most appropriate). Length (using meters would be the most appropriate, kilometres if long enough) Mass (tonnes would be the most appropriate). Volume (m3 would be the most appropriate). Area (m2 would be the most appropriate). Time taken to cross (depends on the length of the bridge. Minutes and seconds would be appropriate).
Meters
Any measure of length will do, but the most common would be feet or meters.
Grams is no good - it is a measure of weight not length. Kilometers measure length - but only if your table-top is a few miles long. Most appropriate would be Meters - your average dining table is about one-and-a-half meters long.
Metres.
TESTOSTERONE
length would be about 1dm, but dms are not used. centimetres would be more mainstream to use
Most UTP cable is limited to 100 meters.
Meters
A standard board is usally 31-33" long. Usally the shorter the skater the shorter the board. The lenght of a skateboard is personal preference. Over time a skater will find out what size board works best for them. It is kinda hard to go wrong with length any standard board is appropriate.
Meters or feet. It is usually meters though. Eg: 2.1M