The answer is a) the light dependent reactions involving photosystems I and II.
During the light-dependent reactions of photosynthesis, which take place in the thylakoid membranes of the chloroplasts, water molecules are split in a process known as photolysis. This reaction occurs in the presence of light and involves both Photosystem I and Photosystem II. The splitting of water releases oxygen (O2) as a byproduct. In contrast, carbon fixation occurs in the Calvin cycle, which does not produce oxygen. The Krebs cycle is part of cellular respiration and is unrelated to photosynthesis. Therefore, the correct answer is that O2 is produced during the light-dependent reactions.