What unique feature defines a Class D fire?

A Class D fire is specifically characterized by the presence of combustible metals such as magnesium, titanium, potassium, and sodium. These metals burn at very high temperatures and react distinctly with water, often causing further hazards when attempted to be extinguished with water. This unique behavior necessitates the use of specialized extinguishing agents that are designed specifically for these types of metals. Such agents may include dry powders like sodium chloride or other chemical compounds that smother the fire without causing violent reactions.

The other features mentioned in the other options pertain to different classes of fires or scenarios. Common combustibles are more applicable to Class A fires, which can be extinguished with water. Flammable liquids refer to Class B fires, while cooking oils are typically associated with Class K fires that focus on kitchen-related hazards. Hence, the requirement of special extinguishing agents for metals distinctly defines Class D fires.