Which of the following reactions is used to estimate Copper volumetrically ?

Question

$ (a)\;2Cu^{2+}+4I^{-} \longrightarrow Cu_2I_2+I_2 \\(b)\;2Cu^{2+}+2CNS^{-}+SO_2+2H_2O \longrightarrow Cu_2(CNS)_2+H_2SO_4+2H^{+}\\(c)\;2Cu^{2+}+4CN^{-} \longrightarrow Cu_2(CN)_2+(CN)_2 \\ (d)\;Cu^{2+}+4NH_3 \longrightarrow [Cu(NH_3)_4]^{2+} $