Why does a block of plastic released under water come up to the surface of water ?

Solution:
Density of plastic block is less than the density of water. Thus, when it floats, it is partially immersed in water such that its weight gets balanced with the buoyant force acting on it. When it is put completely in water, buoyant force on it becomes more than its weight. Thus, when it is released, it comes up to the surface of the water due to a net upward force acting on it.