Correct Answer - A
The presence of small amount of appropriate electrolytes is necessary for the stablility of the colloids. How-ever,when an electrolytic is added in larger concentration,the particles of the take up the ions which which are oppositely charged and thus get neutralized.The neutral particles then start aggregating giving particles of larger size which are then precipitated.
The process of settling of colloidal particles is called coagulation (or precipitation) of the sol.
At lower concentration of electrolytes,the aggregation of particles is called flocculation that can be reversed on shaking while at higher concentration of elecrolyte,coagulation takes place and the same cannot be reversed simply by shaking.
When river water containing colloidal clay flows into sea,elecrolytes present in the brine induce aggregation. This is a major cause of silting in estuaries.
