The correct answer is option 1: water. During photosynthesis, plants use the energy from sunlight to convert carbon dioxide and water into glucose (a form of sugar) and oxygen. This process takes place in the chloroplasts, specifically in a part called the thylakoid membrane.

In the thylakoid membrane, there are pigments called chlorophyll that absorb light energy. This energy is then used to power a series of chemical reactions that separate water molecules into hydrogen ions, electrons, and oxygen. This process is known as photolysis.

The oxygen released during photosynthesis comes from the oxygen atoms in water molecules being split apart. The hydrogen ions and electrons produced by photolysis are used to create energy-rich molecules, which are then used to convert carbon dioxide into glucose through a series of reactions known as the Calvin cycle.

Therefore, it is water that provides the source of oxygen in photosynthesis.