I thought it was a "D" fire, meaning it's a combustible metal fire. most fire suppression equipment is A-B-C compliant (A ordinary combustibles, B flammable liquids, C electrical).

source: I was certified for wildfire fighting and had the basics drilled into me. I can also look at the MSDS diamond and tell you right away how afraid of whatever is inside I am.

edit: fun link https://fireprevention.utexas.edu/firesafety/abcs-fire-extinguishers