The correct answer is option 4, water. Photosynthesis is the process by which plants, algae, and some bacteria convert sunlight, carbon dioxide, and water into glucose (a type of sugar) and oxygen. During this process, water molecules are split into hydrogen ions and oxygen atoms. The hydrogen ions are used to produce glucose, while the oxygen atoms are released as oxygen gas, which is a waste product of photosynthesis.

Option 1, starch, is incorrect because starch is a complex carbohydrate that is produced as a storage form of glucose in plants. Starch is not involved in the production of oxygen during photosynthesis.

Option 2, carbon dioxide, is also incorrect. Carbon dioxide is one of the raw materials needed for photosynthesis, but it does not provide the oxygen needed. Instead, carbon dioxide is used by plants to produce glucose.

Option 3, glucose, is incorrect because glucose is one of the end products of photosynthesis, and it is not involved in providing oxygen during the process.

Therefore, the breakdown of water molecules provides the oxygen needed for photosynthesis.