The real challenge in making stainless steel is stripping out carbon without also burning away the costly chromium that gives the steel its corrosion resistance, since both elements compete for the same oxygen. Blowing in pure oxygen alone would oxidise a lot of chromium into the slag along with the carbon. AOD gets around this by diluting the oxygen with inert argon, which lowers the carbon monoxide partial pressure in the melt and tips the chemical balance in favour of removing carbon while sparing the chromium. This kind of selective decarburisation is exactly what stainless steelmaking needs, and copper, aluminium or zinc processing have no such requirement. So the AOD process is mainly used in stainless steel refining, option (C).