The answer is B.
When lime i.e Ca (OH)2 is added to wastewater that constitute heavy metals,the metals react with hydroxide ions to form insoluble precipitates. This lime is basically added to raise the pH in order to favor the metal precipitation reaction.
After the addition of lime,this is usually followed by the addition of a coagulant and a flocculant wherein alum/ferric salts are added to the negatively charged colloidal metal hydroxide precipitate formed to form a microfloc. This floc settles at the bottom and the liquid at the top layer which is free from these colloidal precipitates is then decanted to a settling tank.
