Which of the following could be a reducing agent or fuel?

A reducing agent is a substance that donates electrons in a chemical reaction, thereby reducing another substance while itself becoming oxidized. In the context of fuels, these substances can also provide energy when they undergo combustion.

Carbon monoxide is a good example of a reducing agent. During combustion, carbon monoxide can react with oxygen to produce carbon dioxide, releasing energy in the process. This makes it an effective fuel and a reducing agent in various chemical reactions, particularly in metallurgy where it can reduce metal oxides to their corresponding metals.

In contrast, the other substances listed do not serve as effective reducing agents or fuels. Heavy metal oxides tend to act as oxidizing agents rather than reducing agents. Hydrogen peroxide, while it can decompose to release oxygen and is sometimes considered in reactions involving reduction, is generally not a typical reducing agent in the same way carbon monoxide is. Concentrated sulfuric acid is a strong acid and an oxidizing agent as well, meaning it does not serve as a reducing agent and is more likely to take electrons rather than donate them.