The correct answer is: B. an oxidising agent.
Carbon dioxide is an oxidising agent because it can accept electrons from other substances. This is because carbon dioxide has a high electronegativity, which means that it is able to attract electrons to itself. When carbon dioxide accepts electrons from other substances, it is reduced, and the other substances are oxidised.
A reducing agent is a substance that can donate electrons to other substances. This is because reducing agents have a low electronegativity, which means that they are able to give electrons away. When a reducing agent donates electrons to other substances, it is oxidised, and the other substances are reduced.
A dehydrating agent is a substance that can remove water from other substances. This is because dehydrating agents have a strong affinity for water molecules. When a dehydrating agent comes into contact with water, it will absorb the water molecules and leave the other substance dry.
A bleaching agent is a substance that can remove colour from other substances. This is because bleaching agents can break down the pigments that give colour to other substances. When a bleaching agent comes into contact with a coloured substance, it will break down the pigments and leave the substance white.