The correct answer is c) liter.
In the metric system, the liter is the standard unit of volume. It is defined as the volume of a cube that measures 10 centimeters on each side, which corresponds to 1,000 cubic centimeters. The liter is commonly used for measuring liquids, and it is widely recognized and accepted around the world.
To clarify the other options: a kilogram is a unit of mass, not volume; a millimeter is a unit of length; and a centimeter is also a unit of length. Therefore, when it comes to measuring volume, the liter is the appropriate choice.