In which class of fires can water be used as an effective extinguishing agent?

Water is effective in extinguishing Class A fires, which involve ordinary combustible materials such as wood, paper, and cloth. This type of fire primarily results from solid materials that can be cooled down and extinguished by the application of water. The water works by absorbing heat and lowering the temperature of the burning material below its ignition point.

In contrast, using water on other classes of fire can be dangerous. For instance, Class B fires involve flammable liquids where water can spread the fire. Class C fires pertain to electrically energized equipment, where water poses a risk of electrical shock. Class D fires involve combustible metals, which can react violently with water.

Therefore, the correct application of water as an extinguishing agent is specifically for Class A fires, making it the right answer in this scenario.