Final answer:
The model represents a chemical reaction.
Step-by-step explanation:
A chemical reaction is represented by a chemical equation, which shows the change from reactants to products. The arrow in the chemical equation separates the reactants from the products and indicates the direction in which the reaction proceeds. In the given model, 2CO + O2 → 2CO2, it represents a chemical reaction where carbon monoxide and oxygen react to form carbon dioxide. Therefore, the correct answer is option a. a chemical reaction.