A red solid is insoluble in water. However, it becomes soluble if some KI is added to water. Heating red solid in a test tube produces violet coloured fumes and droplets of metal appear on the cooler parts of test tube. The red solid is

Question

(a) $(NH_4)_2Cr_2O_7$
(b) $HgI_2$
(c) $HgO$
(d) $Pb_3O_4$