This gas is required for the process of photosynthesis.
Carbon dioxide
The explanation for the incorrect option- (C) Carbon dioxide
Photosynthesis- The process by which plants use carbon dioxide and water in presence of sunlight to form sugar(carbohydrate) and oxygen.
The explanation for the incorrect options-
(A) Nitrogen - It does not take part in the process of photosynthesis.
(B) Hydrogen - It does not take part in the process of photosynthesis.
Hence, option (C) is correct. Carbon dioxide gas is required for the process of photosynthesis.