Final answer:
The most appropriate U.S. Standard System unit for expressing the volume of a bathtub is gallons, as it represents a larger capacity suitable for such containers.
Step-by-step explanation:
When considering the most appropriate unit in the U.S. Standard System for expressing the volume of a bathtub, we should choose a unit that can adequately represent a larger capacity. While pints and liters are units of volume, in the U.S. Standard System, the volume of such a sizable container would be most commonly expressed in gallons. A gallon is equivalent to four quarts, eight pints, and 128 ounces, making it a suitable measurement for the relatively large amount of liquid a bathtub can hold.