The sol particles acquire electrical charge in the following ways:

a) due to dissociation of surface molecules into ions

b) due to selective adsorption of ions from the solution

c) by forming Helmholtz electrical double layer of positive and negative charges on molecule which occurs at the surface.

So all these three phenomenon occurs at the surface only, which occurs during adsorption process.

But in (iv), the process is absorption, which occurs at the bulk of the solution, which is not a surface phenomenon. So in (iv) electrical charge cannot be acquired. Therefore, option (iv) is the correct.