The correct option is
B Respiration.
The green plants carry on the process of photosynthesis. During this process, they take in carbon dioxide from atmosphere and reduce it to sugars. Simultaneously, green plants absorb water from soil and oxidize it to release electrons and oxygen is released as a by product of photosynthesis. On the other hand during respiration, green plants like all the other organisms consume oxygen to oxidise reduced substrates like carbohydrates and release carbon dioxide.Therefore, the correct answer is option B.