Step-by-step explanation:
The correct answer is c. oxygen.
While carbon dioxide, water, and sunlight are all essential for plants to carry out photosynthesis, oxygen is not necessary for the process itself. Oxygen is actually produced as a byproduct of photosynthesis. During photosynthesis, plants use carbon dioxide, water, and sunlight to convert them into glucose (energy-rich organic molecules) and release oxygen as a waste product.
Therefore, the best answer is c. oxygen.