Which law is illustrated in the formation of CO and CO₂? 

Law of Multiple Proportion

The correct answer is option 4. Law of Multiple Proportion.

The law illustrated in the formation of \(CO\) (carbon monoxide) and \(CO_2\) (carbon dioxide) is the Law of Multiple Proportions.

The Law of Multiple Proportions, proposed by John Dalton, states that when two elements form more than one compound, the ratios of the masses of the second element that combine with a fixed mass of the first element can be expressed in small whole numbers. This law helps explain the existence of different compounds between the same elements, each with its own unique ratio of masses. In the case of carbon and oxygen, carbon monoxide \((CO)\) and carbon dioxide \((CO_2)\) are two different compounds formed by the same elements in different ratios.