Final answer:
The unit that does not use metric units to measure the density of a solution is C) Pounds per gallon (lb/gal).
Step-by-step explanation:
Among the given units for measuring the density of a solution, the option that does not use metric units is C) Pounds per gallon (lb/gal). The metric system includes units such as grams, kilograms, and liters. However, pounds and gallons are part of the United States customary units, which are different from the metric system.
The SI unit of density is kilograms per cubic meter (kg/m³), but this can be bulky for daily lab use. Therefore, density for solids and liquids is often expressed in grams per cubic centimeter (g/cm³) or grams per milliliter (g/mL), both of which are metric units. For gases, density is usually measured in grams per liter (g/L), as gases have much lower densities compared to solids and liquids.