Which component of a fire truck carries water for firefighting?

The component of a fire truck that is specifically responsible for carrying water for firefighting is the water tank. The water tank stores a significant volume of water that firefighters can rapidly draw upon when responding to a fire. This allows for immediate access to water, which is crucial for effectively combating flames and preventing the fire from spreading.

In contrast, the water pump is primarily used to move the water from the tank through the hose or nozzle. While the hose reel and fire hose are critical for delivering water to the fire scene, they do not store water themselves. Instead, they are components that facilitate the distribution of the water sourced from the tank. Thus, the water tank is the correct answer, as it is the essential component for holding the water that is utilized during firefighting operations.