Which of the following agents is primarily a dry powder used specifically for class D fires?

The correct agent used specifically for class D fires is potassium chloride. Class D fires involve combustible metals such as magnesium, titanium, and sodium, which require specialized extinguishing agents that can effectively smother or react with the burning materials. Potassium chloride is a dry powder that interrupts the combustion process and extinguishes the fire by forming a barrier between the fuel and the surrounding environment.

Other agents listed do not effectively address class D fires. Halon is primarily used for class B and C fires and is not suitable for combustible metals. Sodium bicarbonate is also not designed specifically for class D fires, even though it has some fire-extinguishing properties. ABC dry chemical extinguishers are versatile but typically do not meet the specific requirements for class D fires, as their formulation is not appropriate for use on reactive metals.