Mac Arthur process is used for

$(a)\;Au \\(b)\;Fe \\(c)\;Cl \\ (d)\;O_2$

Mac Arthur process is used for $Au$
Gold cyanidation (also known as the cyanide process or the MacArthur-Forrest process) is a hydrometallurgical technique for extracting gold from low-grade ore by converting the gold to a water-soluble coordination complex. It is the most commonly used leaching process for gold extraction.
Hence a is the correct answer.
edited Nov 25, 2017