Final answer:
Class D fire refers to fires involving combustible metals like magnesium and titanium, which require special extinguishing agents other than CO₂ extinguishers.
Step-by-step explanation:
The class of fire that involves combustible materials such as magnesium and titanium is known as a Class D fire. These fires involve metals that can burn at high temperatures and are difficult to extinguish with traditional methods. A CO₂ fire extinguisher is ineffective against a magnesium fire because the reaction between magnesium and CO₂ actually generates more combustible material, as shown by the chemical equation 2Mg(s) + CO₂(g) → 2MgO(s) + C(s). Therefore, special extinguishing agents such as dry sand, graphite powder, or a Class D fire extinguisher should be used for these types of fires. Magnesium, known for its use in flares and fireworks, emits a brilliant white light when burned. Titanium is another example of a metal that burns in the form of titanium oxide, which can also be challenging to extinguish with conventional methods.