The correct answer is D) Nonconductivity. The key requirement for any extinguishing agent used on a Class "C" fire (which involves energized electrical equipment) is that it must be non-conductive. This is because the extinguishing agent must not carry electricity, as that could potentially shock the operator or cause further damage to the electrical equipment. The other answer choices, while important properties for other fire classes, are not the critical requirement for Class "C" fires. Extinguishing agents like carbon dioxide or dry chemical powder are commonly used for Class "C" fires due to their non-conductive nature.