Which of the following best defines "usable water"?

Usable water is best defined as water that can support life for various organisms. This includes drinking water for humans and animals, as well as water that sustains aquatic ecosystems. Usable water must be of sufficient quality and availability to meet the needs of different life forms, making it a crucial resource for both environmental health and human consumption.

In differentiating this from the other choices, while water can technically be stored indefinitely (first option), this doesn't inherently define its usability. The underground water reference (third option) is also misleading, as usable water can exist above ground in lakes, rivers, and reservoirs. The last option, which suggests usable water is always clean, does not hold true since some water, even if polluted to a certain degree, may still be functional for certain organisms or purposes. Ultimately, the ability of water to support life is the most comprehensive definition of what makes it usable.