Answer:
The correct option is D. A mathematical model
Explanation:
A mathematical model can be described as a model that describes a system or a cycle based on the concept of mathematics. Pie charts, equations and formulas are mathematical models.
The model shown in the question tells that two molecules of carbon monoxide combine with a single molecule of oxygen to form two molecules of carbon dioxide. The equation is balanced according to the mathematical concepts. Hence, D is the correct answer.