Moll’s half leaf experiment proves that ________ is essential for photosynthesis to take place.
a) chlorophyll
b) carbon dioxide
c)light
d)water

Hint: Photosynthesis is the process in which plants make the food when they derive energy from the sunlight. It is important to process for the survival of the plants. Plants use sunlight and gas used for respiration during the process of photosynthesis.

Complete answer:
Molls half leaf experiment was done to prove that plants cannot do photosynthesis in the absence of the carbon dioxide. Plants need light and carbon dioxide as their essential component for the process of photosynthesis. If either of them is absent the whole system is affected. In Moll's experiment a plant was taken and was made to de starch completely. This took two days’ time. Then a bottle partially filled with a strong solution of the caustic potash was taken and its mouth was closed from a cork. Half a portion of the leaf was then inserted into the bottle through the split cork. Now the whole apparatus was put in sunlight after the application of grease on the upper portion of the split cork. Then after 10hours starch test was done on the leaf of the plant. The leaf which was present inside the bottle showed negative results as it was deprived of the carbon dioxide. And the leaf present outside the bottle showed the presence of starch. These results indicated that the presence of carbon dioxide is necessary for the plant to carry out photosynthesis successfully.
So, the answer is ‘carbon dioxide’.

Note: In the experiment, the leaf present inside the bottle was deprived of only carbon dioxide, the sunlight, chlorophyll and water were available to the leaf. The leaf that showed the positive results wasn’t deprived of the carbon dioxide that is why the process of starch making occurred in the leaves giving positive results.