The correct option is A Oxygen
Photosynthesis is the process by which green plants make their own food from carbon dioxide and water in the presence of sunlight. The prepared food is glucose. To prepare one molecule of glucose, by the process of photosynthesis six molecules of carbon dioxide and six molecules of water is required. Six oxygen molecule is released as a waste product. This oxygen is produced due to splitting of water by the process of photolysis during light reaction. So, the correct answer is option 'Oxygen'.