The volume of the balloon would decrease if you moved it from above a water surface to ten feet below the water surface.
This is because the water pressure increases with depth. At 10 feet below the water surface, the water pressure is about 33.4 feet of water (33.4 psi). This pressure pushes inward on the balloon, causing it to shrink.
The balloon would not explode because the air inside the balloon is compressed. The air molecules are forced closer together, but they do not have enough energy to escape from the balloon.
If the balloon was not tied, the increase in water pressure would cause the balloon to expand and eventually burst.
The other options are incorrect. The balloon would not explode, and the volume of the balloon would not increase or stay the same.