Introduction:
In chemistry, a redox reaction is a type of chemical reaction that involves the transfer of electrons between two species. The term "redox" is derived from the words reduction and oxidation, which represent the two half-reactions that occur in a redox reaction. Reduction involves the gain of electrons, while oxidation involves the loss of electrons. In a redox reaction, one species is oxidized while another species is reduced. Redox reactions are characterized by the change in the oxidation state of the species involved.

Explanation:
Let's analyze each of the given reactions to determine whether they are redox reactions or not.

a) 2NaH ? 2Na + H2:
In this reaction, sodium hydride (NaH) is decomposed into sodium (Na) and hydrogen gas (H2). The sodium atoms are oxidized from an oxidation state of -1 in NaH to 0 in Na, while the hydrogen atoms are reduced from an oxidation state of -1 in NaH to 0 in H2. Therefore, this reaction is a redox reaction.

b) CaCO3 ? CaO + CO2:
In this reaction, calcium carbonate (CaCO3) is decomposed into calcium oxide (CaO) and carbon dioxide gas (CO2). The oxidation state of calcium remains the same throughout the reaction (+2), and the oxidation state of carbon also remains the same (+4). Therefore, this reaction is not a redox reaction.

c) H2 + Cl2 ? 2HCl:
In this reaction, hydrogen gas (H2) reacts with chlorine gas (Cl2) to form hydrogen chloride (HCl). The hydrogen atoms are oxidized from an oxidation state of 0 in H2 to +1 in HCl, while the chlorine atoms are reduced from an oxidation state of 0 in Cl2 to -1 in HCl. Therefore, this reaction is a redox reaction.

d) CuSO4 + Zn ? Cu + ZnSO4:
In this reaction, copper sulfate (CuSO4) reacts with zinc (Zn) to form copper (Cu) and zinc sulfate (ZnSO4). The copper atoms are reduced from an oxidation state of +2 in CuSO4 to 0 in Cu, while the zinc atoms are oxidized from an oxidation state of 0 in Zn to +2 in ZnSO4. Therefore, this reaction is a redox reaction.

Conclusion:
Among the given reactions, the reaction CaCO3 ? CaO + CO2 is not a redox reaction. This is because the oxidation states of the elements involved in the reaction (calcium and carbon) remain the same throughout the reaction.