What is the best method for suppressing a Class D fire?

The best method for suppressing a Class D fire is indeed smothering the fire. Class D fires involve combustible metals such as magnesium, titanium, or sodium, which have unique combustion characteristics and can react violently to water and other common extinguishing agents.

Smothering a Class D fire works effectively because it eliminates the oxygen that fuels the fire. This can be done by covering the fire with a suitable material that prevents oxygen from reaching the combustion. In many cases, dry powder extinguishing agents specifically designed for metal fires can also serve as a smothering agent. These powders not only help to smother the fire but also can absorb the heat from the burning material.

Using water or foam, as mentioned in the other choices, can lead to dangerous reactions and escalation of the fire due to the chemical nature of the burning metals. CO2 extinguishers might not be effective due to the high temperatures involved, which can limit the efficacy of CO2. While dry chemical extinguishers can sometimes be effective for Class D fires, they are not universally recommended for all types of combustible metals, making smothering the more reliable method overall.