Which of the following reactions in a redox reaction?

a
2 Mg(s) + O2(g) → 2 MgO(aq)

b
HCl(aq) + NaOH(aq) → H2O(l) + NaCl(aq)

c
Pb(NO3)2(aq) + KCl(aq) → PbCl2(aq) + KNO3(aq)

d
CaCO3(s) → CaO(s) + CO2(g)