During photosynthesis, green plants absorb carbon dioxide. This is a vital process for plants, as they use carbon dioxide along with sunlight and water to create glucose (a type of sugar) and oxygen. The process of photosynthesis occurs in the chloroplasts of the plant`s cells, specifically in the chlorophyll molecules.

Option 1, nitrogen, is not absorbed during photosynthesis. Plants primarily obtain nitrogen from the soil and use it for various growth processes, but it is not directly involved in photosynthesis.

Option 3, carbon monoxide, is a toxic gas that is not involved in photosynthesis. In fact, carbon monoxide is harmful to plants, as it interferes with their ability to take in carbon dioxide.

Option 4, oxygen, is actually produced as a byproduct of photosynthesis. Oxygen is released into the atmosphere during this process and is used by living organisms for respiration.

Therefore, the correct answer is option 2: carbon dioxide.