Option 1 states that chemical energy is converted into light energy. However, this is incorrect. In the process of photosynthesis, light energy is converted into chemical energy, not the other way around.

Option 2 states that carbon dioxide is oxidized to form carbohydrates. This is partially correct. During photosynthesis, carbon dioxide is indeed converted into carbohydrates through a series of chemical reactions. However, it is important to note that oxidation is not the only process involved. Photosynthesis actually involves both oxidation and reduction reactions.

Option 3, which is the correct answer, states that a water molecule splits into hydrogen and oxygen. This is known as photolysis, or the splitting of water molecules by light energy during photosynthesis. The hydrogen released from water is used to combine with carbon dioxide to produce carbohydrates, while oxygen is released as a byproduct.

Option 4 states that light energy is directly used to split water. This is not entirely accurate. Light energy is required to initiate the process of photolysis, but it is not directly involved in the actual splitting of water molecules.

In conclusion, option 3 is the correct statement about the process of photosynthesis – water molecule splits into hydrogen and oxygen.