Oxygen and hydrogen both exists as gaseous molecules in the atmosphere. However, one of them (oxygen) is a product of photosynthesis while the other (hydrogen) is one of the constituents of the molecule that starts the process of photosynthesis.
According to this question, the X used in the attached diagram is the role of oxygen in photosynthesis. Oxygen is released as a product of photosynthesis and goes out of the plant into the air via the STOMATA. Hence, the gas released into the air via stomata is OXYGEN.
