Which of the following is a standard measure of density?

Density is defined as mass per unit volume. In standard scientific practice, measurements of density are typically expressed in familiar units, such as grams per cubic centimeter (g/cm³) or kilograms per cubic meter (kg/m³).

The correct choice of pounds per gallon is a recognized measure commonly used in industries that deal with fluids, especially in the United States. This unit effectively conveys how much mass (in pounds) is contained in a specified volume (in gallons), making it practical for applications in chemical and physical operations.

The other units provided, while they may relate to measurements of volume or mass, do not represent standard density measures. For example, gallons per cubic foot, milliliters per liter, and liters per milligram do not appropriately express the mass-to-volume ratio that defines density. Therefore, choosing pounds per gallon aligns with conventional methods of expressing and understanding density in the relevant contexts.