@Tracy737
There isn't a scientific answer to what to select.
Keep in mind, on flexible a guest can book your place as a back-up, see a cheaper last minute alternative and then bale out on you. Unless you have an eager queue of last minute bookers, it's a lemon.
You can start out on moderate and see how it goes for you. If you find there are continuous cancellations you can be minded to flip over to strict.
Some say with strict policy, you may get more reliable guests who are committed and stick to their plans.
With moderate, some argue you perhaps attract more bookings this way.
It's trial and error, plus you should consider the seasonal aspect of your listing as well.