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During photosynthesis, the source of oxygen released is water. This process occurs in the chloroplasts of plant cells, where chlorophyll captures light energy, typically from sunlight. In the light-dependent reactions of photosynthesis, water molecules are split through a process called photolysis. This splitting generates oxygen as a byproduct, which is then released into the atmosphere.

The water molecules provide the necessary electrons and protons required for the photosynthetic process. The oxygen released during this reaction is fundamentally important, as it contributes to the oxygen supply in the earth's atmosphere, supporting life by enabling aerobic respiration in animals and other organisms.

The other options do not serve as direct sources of the oxygen released in photosynthesis. Carbon dioxide serves as the carbon source for producing glucose but does not release oxygen. Glucose is a product of photosynthesis and is not involved in the release of oxygen. Oxygen gas is a product of the process but is not a source; rather, it is the end result of the splitting of water molecules.