Which of the following is NOT a base unit in the metric system?

In the metric system, base units are defined as the fundamental units of measurement that are used to define other units. The base units include the meter for length, kilogram for mass, second for time, ampere for electric current, kelvin for temperature, mole for the amount of substance, and candela for luminous intensity.

The liter, while commonly used to measure volume, is actually a derived unit. It is defined as a cubic decimeter (1 dm³), which illustrates that it is based on the meter, a base unit. Thus, the liter does not stand alone as a base unit in the metric system.

The second and meter are both base units; the second measures time and the meter measures length. The gram, although typically considered a common unit of mass, is derived from the kilogram (1 kg = 1000 g), which is the true base unit for mass. Therefore, among the options presented, the liter is not a base unit in the metric system, distinguishing it from the other choices.