What is the standard unit of volume in the metric system?

The standard unit of volume in the metric system is the liter. The liter is defined as the volume of a cube that measures 10 centimeters on each side, which is equivalent to 1,000 cubic centimeters (cm³). It is a base unit in the International System of Units (SI) and is widely used in various scientific and practical applications, including measuring liquids.

In contrast, while milliliters are also used for measuring smaller volumes and are derived from the liter (1 liter equals 1,000 milliliters), the liter itself is considered the primary unit of volume. The centimeter is a unit of length, not volume, and the gallon is an imperial unit of volume used primarily in the United States and some other regions, but it is not part of the metric system. Therefore, the liter is recognized as the fundamental unit of volume in metric terms, making it the correct choice in this context.