Plants  release _______ through the process of photosynthesis   

Plants release O₂ (oxygen) through the process of photosynthesis.

Plants takes up the carbon dioxide from the atmosphere and water from the soil to produce food and release oxygen as a byproduct during the process of photosynthesis. This process takes place in the presence of sunlight hence occurs only during the day. On the other hand, they also utilise oxygen and release carbon dioxide back in the atmosphere during the process of respiration.