Final answer:
Acre-feet is a unit that represents the volume of water required to cover one acre of land to a depth of one foot. This is a three-dimensional measurement, making option A correct. When measuring water for a pool or the content of a reservoir, volume metrics like gallons or acre-feet are used.
Step-by-step explanation:
The amount of water in reservoirs can be measured in acre-feet, which represents a unit of volume. An acre-foot is defined as the volume of water that would cover an area of one acre to a depth of one foot. Since this unit is concerned with both a two-dimensional area (the acre) and a one-dimensional measurement (the foot of depth), the result is a three-dimensional measure of volume.
Considering Darcy filling up her swimming pool, she would likely use gallons to measure the capacity, as gallons are a conventional unit for measuring large fluid volumes in the American system. When calculating the mass of water in a reservoir, like the example of a reservoir with a surface area of 50.0 km² and an average depth of 40.0 m, one would first calculate the volume and then use the density of water to find the mass.