The base unit used to measure length in the metric system is

meter

Step-by-step explanation:

As, we know that the units to measure lengths in a metric system are:

meter, centimeter, millimeter and kilometer.

Out of these four,

The basic unit to measure the length in a metric system is 'meter' represented by 'm'.

Others are the derived units from meter i. e. they are not the base units.

Hence, the base unit used to measure the length in a metric system is meter.

the answers is meters