To answer this you will need to determine which area made with the 12 square tiles would give the shortest perimeter.
Think of all the combinations to get 12 square inches
1x12, 2x6, and 3x4. The perimeters of these are 26 in, 16 in, and 14 in.
You should arrange your tiles using the 3 x 4 option. You would need 14 stones.
