Which agent is NOT recommended for use on Class D fires?

Class D fires involve combustible metals such as magnesium, titanium, and sodium, and require specific methods for extinguishment. Using water on a Class D fire can be incredibly dangerous as it may react violently with certain metals, potentially causing the fire to spread or even explode. This risk is consistent across various metal fires, where water can enhance the combustion process instead of mitigating it.

In contrast, the recommended agents for extinguishing Class D fires, such as dry powders and coarse sand, can effectively smother the flames without creating hazardous reactions. Dry powder agents are specifically designed for metal fires, while coarse sand can help to cover the burning metal and deprive it of oxygen.

In this context, foam is also not suitable, as it is generally effective for Class A and B fires but does not address the unique characteristics of Class D fires, making it inadvisable. Thus, water stands out as the primary agent that should never be used on Class D fires due to its potential to exacerbate the situation.