Which is a redox reaction?

A.     ${{\text{[Cu(}}{{\text{H}}_{\text{2}}}{\text{O}}{{\text{)}}_{\text{4}}}{\text{]}}^{2 + }}{\text{(aq)}} + {\text{4C}}{{\text{l}}^ - }{\text{(aq)}} \to {{\text{[CuC}}{{\text{l}}_{\text{4}}}{\text{]}}^{2 - }}{\text{(aq)}} + {\text{4}}{{\text{H}}_{\text{2}}}{\text{O(l)}}$

B.     ${\text{A}}{{\text{g}}^ + }{\text{(aq)}} + {\text{C}}{{\text{l}}^ - }{\text{(aq)}} \to {\text{AgCl(s)}}$

C.     ${\text{Zn(s)}} + {\text{2HCl(aq)}} \to {\text{ZnC}}{{\text{l}}_{\text{2}}}{\text{(aq)}} + {{\text{H}}_{\text{2}}}{\text{(g)}}$

D.     ${\text{2}}{{\text{K}}_{\text{2}}}{\text{Cr}}{{\text{O}}_{\text{4}}}{\text{(aq)}} + {\text{2HCl(aq)}} \to {{\text{K}}_{\text{2}}}{\text{C}}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{7}}}{\text{(aq)}} + {{\text{H}}_{\text{2}}}{\text{O(l)}} + {\text{2KCl(aq)}}$

C