(3) is right. The water partially supports the solid ball, exerting an upwards force equal to the weight of water displaced, which is 40 g if we take the density of water as 1 g per cm^3. So you can think of it as a case where the left side of the balance received this amount of extra weight.
It you want to think it through more fully, then, as you correctly say, this force on the ball is caused by the pressure difference on the upper and lower surfaces of the immersed ball. But notice that there is also this same amount of force acting downwards on the water! That might seem like an odd way of putting it, but it is what Newton's third law (action and reaction) says. Since the water is not accelerating, the beaker must be providing this amount of extra upwards force at the bottom surface of the water, in order to support the water. Hence the answer to the question.