Which one of the following substances is not a source of 'A' class fire?

Fire classes categorize fires based on the type of fuel involved. Class A fires involve ordinary combustible materials that leave an ash residue, such as wood, paper, cloth, rubber, and various fibrous materials like jute. These fires are typically extinguished using water or multipurpose dry chemical agents.

Stainless steel is a metal. Fires involving combustible metals (such as magnesium, sodium, or potassium) are categorized as Class D fires. Stainless steel is generally non-combustible under standard conditions and does not fall under the Class A category of ordinary combustibles.