The gas released during photosynthesis in plants is :

Oxygen is released during the process of photosynthesis. Photosynthesis is the process in which green plants produce glucose. It involves the conversion of carbon dioxide gas, water, and minerals the plants take in from their surroundings into sugar and gaseous oxygen. Photosynthesis involves the intake of carbon dioxide and the release of oxygen. The release of oxygen is due to photolysis of water in order to fill the gap in the photosystem created by the excitement of electron. 

So, the correct answer is option B.