Lyophobic sols are unstable and can be easily coagulated by adding electrolyte. This can be avoided by adding lyophilic sol which forms a thin film or protective layer.So option (iii) is correct.
Addition of oppositely charged sol also results in precipitation of lyophobic sol by the interaction between two oppositely charged ions. So (i) is wrong.
On boiling, collisions occur between lyophobic sol and dispersion medium particles. This results the reduction of a charge of sol particles and ultimately settle down as precipitate. So (iv) is also wrong.
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