Which of the following is a unit of mass?

The unit of mass in the metric system is the gram. It is commonly used to measure the mass or weight of various substances. One gram is defined as one one-thousandth of a kilogram, which is the base unit for mass in the International System of Units (SI).

The other options listed represent different types of measurements. A meter measures length, which is the distance between two points. A liter is a unit of volume, used to quantify how much space a substance occupies, often used with liquids. Finally, a millimeter is another unit of length, specifically one thousandth of a meter. Therefore, the choice of gram as the unit of mass is accurate and reflects the standard measurement used in scientific contexts.