Which of the following base units includes a prefix?

The kilogram is the correct choice because it incorporates a prefix, specifically "kilo-," which denotes a factor of one thousand (10^3). The prefix modifies the base unit "gram," which is one of the standard units of mass in the metric system. In the International System of Units (SI), base units do not have prefixes; thus, the gram is a base unit by itself without a prefix.

The other units listed—meter, gram, and liter—are all base units themselves without any prefix. The meter is the standard unit of length, the gram is the standard unit of mass (without the kilo- prefix), and the liter, which is commonly used to measure volume, is also considered a derived unit based on the cubic decimeter but does not have a prefix in its standard form.

Therefore, the inclusion of the "kilo-" prefix in the kilogram sets it apart as the only base unit among the options that includes a prefix.