Explanation:

An oxidizing agent is a substance that accepts electrons from another substance during a redox reaction. It gets reduced, meaning it gains electrons and is usually left with a more positive oxidation state.

Options:
a) sodium hydride: Sodium hydride (NaH) is a reducing agent, not an oxidizing agent. It donates electrons and gets oxidized, meaning it loses electrons during a redox reaction. Therefore, option (A) is the correct answer.

b) carbon dioxide: Carbon dioxide (CO2) is not an oxidizing agent. It is a nonpolar molecule composed of two oxygen atoms double bonded to a carbon atom. It does not have the capability to accept or donate electrons during a redox reaction.

c) ozone: Ozone (O3) is an oxidizing agent. It is a highly reactive molecule composed of three oxygen atoms. It readily accepts electrons from other substances, causing oxidation. Ozone is commonly used as a disinfectant and a bleaching agent.

d) magnesium oxide: Magnesium oxide (MgO) is not an oxidizing agent. It is an ionic compound composed of magnesium cations (Mg2+) and oxide anions (O2-). It does not undergo any redox reactions as the oxidation state of both magnesium and oxygen remains the same.

Summary:
Sodium hydride (NaH) is the only option that does not act as an oxidizing agent. Carbon dioxide (CO2), ozone (O3), and magnesium oxide (MgO) can all behave as oxidizing agents in different chemical reactions.