D. All of the above can cause someone's weight to fluctuate, even when following a strict protocol.
Water weight or retention can cause temporary weight gain or loss due to changes in hydration levels or hormone fluctuations. The female menstrual cycle can cause weight fluctuations due to hormonal changes and water retention. The digestive cycle can cause temporary fluctuations in weight due to the digestion and elimination of food. It's important to understand that these fluctuations are normal and may not be a reflection of overall fat loss or gain. Therefore, it's essential to focus on long-term trends rather than short-term fluctuations in weight when evaluating progress towards weight loss or weight gain goals.