Explanation:

Mohl's half experiment is a technique to prove that carbon dioxide is necessary for photosynthesis. The experiment involves splitting a leaf into two halves and exposing one half to carbon dioxide and the other half to oxygen. The experiment is performed in a bell jar where the leaf is exposed to light.

The following points explain the experiment and the results:

- Preparation: A potted plant is kept in a bell jar and a leaf is selected for the experiment. The leaf is split into two halves with a sharp blade, making sure that the midrib is not cut. One half is exposed to carbon dioxide while the other half is exposed to oxygen. The bell jar is sealed to prevent air from escaping.

- Observation: After a few hours, the leaf exposed to carbon dioxide releases oxygen bubbles while the other half does not release any bubbles. This is because the half exposed to carbon dioxide undergoes photosynthesis, which produces oxygen as a byproduct. The half exposed to oxygen does not undergo photosynthesis and hence does not produce any oxygen.

- Conclusion: The experiment proves that carbon dioxide is necessary for photosynthesis. This is because the half exposed to carbon dioxide produces oxygen, which is a byproduct of photosynthesis. The half exposed to oxygen does not produce any oxygen because it cannot undergo photosynthesis without carbon dioxide.

Therefore, option C (carbon dioxide is necessary) is the correct answer.