Water gas is a mixture of carbon monoxide (CO) and hydrogen (H2), which makes option 1 the correct answer.

To understand why, it is important to know the composition of water gas. Water gas is produced by passing steam over red-hot coke or coal in a process called water gas shift reaction. This reaction produces carbon monoxide and hydrogen gas.

Option 1 mentions a mixture of carbon monoxide and hydrogen. This fits the definition of water gas since it is indeed a combination of these two gases.

Option 2 mentions a mixture of carbon monoxide and nitrogen. Nitrogen is not a component of water gas, so this option can be eliminated.

Option 3 mentions a mixture of carbon dioxide and water vapor. While carbon dioxide may be present in small amounts in water gas, it is not the main component. Additionally, water vapor alone does not constitute water gas. Hence, this option is also incorrect.

Option 4 mentions a mixture of carbon monoxide and water vapor. While there may be traces of water vapor in water gas, it is not the predominant component. Therefore, option 4 is not the correct answer.

In conclusion, the correct answer is option 1, a mixture of carbon monoxide and hydrogen