Carbon and oxygen combine to form carbon dioxide. What are the reactants, or what is the reactant, in this chemical reaction?
carbon dioxide
carbon only
oxygen only
carbon and oxygen

The correct option is d: carbon and oxygen.

Explanation:

The reaction between carbon and oxygen to form carbon dioxide is:

C + O₂ → CO₂    (1)

On the left side of the above reaction (1), we have the reactants, which are carbon (C) and oxygen (O₂) that react to form carbon dioxide (CO₂), which is the only product of the reaction (on the right side of the reaction).      

Hence, the correct option is d: carbon and oxygen.

I hope it helps you!