ANS:B - photochemical reaction.

Plants produce carbohydrates from the CO2 present in the atmosphere by photochemical reaction. Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy stored in the bonds of carbohydrates (such as glucose). This process occurs in chloroplasts, specialized organelles found in plant cells. During photosynthesis, chlorophyll molecules within the chloroplasts absorb light energy from the sun. This light energy is used to power a series of photochemical reactions that split water molecules (H2O) into oxygen (O2) and hydrogen ions (H+). The hydrogen ions and electrons generated from water splitting are used to reduce carbon dioxide (CO2) to carbohydrates, such as glucose, through a series of enzymatic reactions known as the Calvin cycle. In summary, plants utilize a photochemical reaction during photosynthesis to convert CO2 from the atmosphere into carbohydrates, using light energy absorbed by chlorophyll molecules. Therefore, the correct answer is a photochemical reaction.