The reaction that involves a change in oxidation states is a redox reaction. Let's examine each option:
a) AgNO3 + NaCl → AgCl + NaNO3: This reaction involves the exchange of ions but does not involve a change in oxidation states. Therefore, it is not a redox reaction.
b) H2O → H2 + O2: This reaction involves the decomposition of water into hydrogen and oxygen gases. There is a change in oxidation states from -2 in water to 0 in hydrogen gas and +2 in oxygen gas. Therefore, this is a redox reaction.
c) NaOH + HCl → NaCl + H2O: This reaction involves the exchange of ions but does not involve a change in oxidation states. Therefore, it is not a redox reaction.
d) CaCO3 → CaO + CO2: This reaction involves the decomposition of calcium carbonate into calcium oxide and carbon dioxide. There is no change in oxidation states; calcium has an oxidation state of +2 in all compounds involved, and carbon has an oxidation state of +4 in all compounds involved. Therefore, this is not a redox reaction.
So, the redox reaction among the options is:
b) H2O → H2 + O2
