Purple of cassius is a/an: colloidal sol of silver, oxyacid of gold, colloidal sol of platinum, colloidal sol of gold

Purple of Cassius is a purple pigment formed by the reaction of gold salts with tin(II) chloride. It has been used to impart glass with a red colouration (see cranberry glass), as well as to determine the presence of gold as a chemical test. It is a colloidal sol of gold. Hence, option A is correct.