Which of the following best describes the purpose of a water can in firefighting?

The primary function of a water can in firefighting is to supply water for small fires. This tool is designed to allow firefighters to quickly and efficiently extinguish minor fires, particularly in settings where a fire hose may be less practical to deploy. Water cans are often equipped with a nozzle that helps in directing the flow of water precisely where it's needed, making them very effective for controlling small blazes or spot fires.

Other options, such as holding excess fire tools, storing oxygen, or providing structural support, do not align with the specific use of a water can and its essential role in firefighting strategy. These functions are typically carried out by different types of equipment or apparatus designed specifically for those purposes.