What type of extinguishing agent is primarily used in Class A fire extinguishers?

Class A fire extinguishers are specifically designed to extinguish fires that involve ordinary combustible materials such as wood, paper, cloth, and various types of trash. The primary extinguishing agent used in Class A fire extinguishers is water. Water effectively cools down the burning materials and helps to smother the flames by removing the heat element of the fire tetrahedron.

Using water is especially effective for Class A fires because it can absorb significant amounts of heat, and when applied, it helps convert water to steam, which can further displace oxygen around the fire, assisting in extinguishing the flames. For this reason, water is the most suitable and commonly used extinguishing agent for Class A fires, making it the correct answer in this context.

Other agents like foam, CO2, and dry chemical serve different purposes and are more suited for different classes of fires. Foam is typically used for Class B fires (flammable liquids), CO2 is effective for electrical and Class B fires, and dry chemical can be used for various fire classes but may not be as effective specifically for Class A materials as water is.