A is my favorite, for a couple of reasons. I like the Borealis there, but I also think the water works very well as a reinforcer of the calming influence you're looking for. I don't get the same sense from B, and there's nothing immediately apparent to differentiate C from D. If I were picking up a book in the store--or sorting through the blizzard of meditation books online--A would catch my attention better than the others would.

You cannot work too hard on the cover. In today's publishing environment, it's the make or break of a book, probably even more than the reviews it gets, though those matter a great deal. What you have here is a good, solid cover that speaks to professionalism and argues that you probably know what you're doing with the material between the pages. Godspeed.