Correct option is (b) Convex Side
To explain I would say: Velocities at the bottom of the bend are much less than the velocities at the top, and enough centrifugal force is not present to counteract this tendency of water at the top. Due to this the water dives in from the top at the concave side and moves at the bottom towards the convex side. These rotary currents cause erosion at the concave side and deposition at the convex side, forming shoals on the convex side.