Final answer:
Carbon monoxide (CO) is the best classified as a compound because it contains two different types of atoms, carbon and oxygen.
Step-by-step explanation:
A compound is a substance that contains two or more elements chemically combined in a fixed proportion. Compounds can be classified based on their composition. In the options provided, carbon monoxide (CO) is the best classified as a compound because it contains two different types of atoms, carbon and oxygen.