Flexibility- sitting


Hi! I'm struggling with lower body flexibility and would like some advice. I want to be able to sit comfortably on the floor with my legs straight out. Here are some pics of me sitting in progressively lowers positions...you can see the way my back posture gets more hunched in each one. How do I improve this? I'm not sure which muscles to work on. If anyone can suggest specific exercises/stretches I'd be very grateful!