So, it's all based on the number of stones in each ring. Top left-has 7, top right has 3, bottom left has 5 and bottom right has 1.

Hint #1 All are prime numbers. None of the rings are eliminated.

HInt #2 Subtract one stone from each and all are no longer prime numbers (even zero, about which some mathematicians like to argue). None are eliminated.

Hint #3 After subtracting the one, the rings have 6, 2, 4 and 0 stones. The mean of the four is now 3 (12 divided by 4) which is equal to the top right if you added back the one. So top right is eliminated.

Hint #4 Adding back the one to the other three gives you 7, 5 and 1 stones. Subtracting the 5 from the 7 gives you 2, which is twice 1 (same as saying 1 is half of 2), so that eliminates the bottom right.

Hint #5 The square of 7 is 49 and the square of 5 is 25. Subtracting 25 from 49 leaves you with 24. The total stones in all four rings was 16, so 24-16=8.

Hint #6 One less than 8 is 7. So, my favorite is the top left.