Explanation:

In all physical processes taking place in closed systems, the amount of change in kinetic energy is equal to the amount of change in potential energy. If the kinetic energy increases, the potential energy decreases, and vice-versa.

When the height is increasing, P.E will increase (on moving a body against gravitational force) and when the height is decreasing, P.E will decrease (on free fall of an object). On rising of an air bubble in water, depth of bubble starts decreasing which lead to decrease in its buoyancy potential energy. So , answer is 4.