Which equation represents and oxidation/reduction (redox) reaction?

a) ZnS(s) + 2HCl(aq) → ZnCl2(aq) + H2S(g)

b) H2SO4(aq) + 2KOH(aq) → K2SO4(aq) + 2H2O(l)

c) 2Al(NO3)3(aq) + 3Zn(s) → 2Al(s) + 3Zn(NO3)2(aq)

d) Na2CO3(aq) + Ca(NO3)2(aq) → CaCO3(s) + 2NaNO3(aq)