The correct answer is option 2: Water. During photosynthesis, oxygen is produced as a byproduct when water molecules are split apart. This process is known as photolysis, which occurs in the light-dependent reactions of photosynthesis. In these reactions, light energy is absorbed by chlorophyll in the chloroplasts of plant cells. This energy is then used to split water molecules into oxygen gas (O2), protons, and electrons. The oxygen atoms from the water molecules combine to form molecular oxygen (O2), which is released into the atmosphere as a waste product. Therefore, the source of the oxygen liberated during photosynthesis is water, not carbon dioxide, breakdown of chlorophyll, or the atmosphere.