Water, sparkling water, tea, and coffee without any sweeteners or other additives, artificial or otherwise.
However, here’s what I tell folks as a rule of thumb. If you’re fasting for health reasons and trying to hit specific physiological states, water only - some folks will say tea and black coffee as well. If you’re fasting as a caloric reduction to lose weight, most very low or calorie free options can be worth it if they help get you to your next meal. But it’s something to personally experiment with.
For instance, some folks have great results still chugging diet soda, I found cutting it helped eliminate a lot of my snacking hunger.