Explanation:

- Option 1: CO2, CO, and N2

- Incorrect. Marsh gas primarily consists of methane, not carbon dioxide (CO2), carbon monoxide (CO), or nitrogen (N2).

- Option 2: CH4 and H2

- Incorrect. Though hydrogen (H2) can be produced in anaerobic environments, marsh gas specifically refers to methane (CH4).

- Option 3: CH4

- Correct Answer. Marsh gas is primarily composed of methane (CH4). It is produced in anaerobic conditions, such as marshes, by microbial decomposition of organic matter.

- Option 4: CO

- Incorrect. Carbon monoxide (CO) is not typically found in marsh gas and is a result of incomplete combustion, not microbial activity in marshes.

The correct answer is Option 3: CH4.