During photosynthesis, plants absorb carbon dioxide (CO2) from the atmosphere and release oxygen (O2) as a byproduct. Therefore, option 2, oxygen, is the correct answer.

Option 1, carbon dioxide, is incorrect because it is the gas that is absorbed by plants, not released. Carbon dioxide is essential for the process of photosynthesis as it provides carbon atoms that are used to build glucose molecules.

Option 3, hydrogen, is also incorrect. While hydrogen is involved in photosynthesis, it is not released as a gas. It is used in the production of energy-rich molecules such as ATP and NADPH, which are essential for the conversion of carbon dioxide into glucose.

Option 4, methane, is incorrect as well. Methane is not released during photosynthesis. Methane is mainly produced by certain bacteria in anaerobic environments and is a potent greenhouse gas.

In conclusion, during photosynthesis, plants release oxygen as a byproduct, making option 2 the correct answer.