In an aqueous solution of Fe (III) and (SCN−) , the equilibrium can be represented as follows: Fe3+(aq)PALE    YELLOW+SCN−(aq)COLOURLESS⇌[Fe(NCS)]2+RED(aq) For this choose correct statements

On addition of oxalic acid (C2H2O4) a stable complex of Fe3+[Fe(C2O4)3]3− is formed and it results in decrease of [Fe3+] in solution. Due to this the reaction shifts backwards and there is a decrease in red colour.  On addition of HgCl2(aq),Hg2+ combines with SCN− and forms [Hg(SCN)4]2− which shifts the equilibrium backwards.