Which substance can act as a reducing agent or fuel?

Carbon monoxide is a substance that can act as a reducing agent or fuel primarily due to its ability to donate electrons in chemical reactions. In redox (reduction-oxidation) reactions, a reducing agent is defined as a substance that can provide electrons to another substance, facilitating its reduction while itself getting oxidized.

In the case of carbon monoxide, it readily reacts with oxygen, often serving as a fuel in combustion reactions. When it reacts with oxygen, carbon monoxide is oxidized to carbon dioxide while simultaneously reducing the oxygen. This characteristic makes carbon monoxide valuable in various industrial processes, including in the production of iron in blast furnaces where it reduces iron ore.

On the other hand, hydrogen peroxide primarily acts as an oxidizing agent rather than a reducing agent, since it can accept electrons. Sodium chloride is a stable ionic compound and does not function as a reducing agent under normal conditions. Oxygen itself is usually an oxidizer, as it tends to accept electrons in reactions rather than donate them.

Thus, carbon monoxide stands out as the correct choice because of its dual ability to act as both a reducing agent and a fuel in various reactions.