Aunit of length in the metric system is the a-kilogram b-kilometer c- liter

Remember that measuring length is a measure of distance. Let's check our answers for any pending measures of distance.

Kilogram.
Kilogram is a measure of mass (or weight). This can normally be used for example for the SI Unit symbol: Kg.

We can get rid of kilogram, since it does not measure distance.

Kilometer.
Kilometer is a measure of distance. This can normally be used for example by it's SI Unit symbol: KM.
Kilometer can be broken down by it's prefix.
Kilo = Thousand

Kilometer means 1,000 meters, which is a unit of distance/length.

Your answer is B.) Kilometer.

I hope this helps!