Green plants continuously require and use carbon dioxide, primarily during the process of photosynthesis, where they convert carbon dioxide and water into glucose and oxygen using sunlight. Therefore, the best answer is b) carbon dioxide, only.
While plants do also produce oxygen during photosynthesis, they do not continuously require it in the same manner as they require carbon dioxide for photosynthesis. Additionally, nitrogen is needed by plants, but it is not required continually or used in the same process as photosynthesis.