Answer: A. Decreases
When alum mixed with water then pH value of water decreases.
When alum (Al2(SO4)3.18H2O) is mixed with water, it reacts with alkalinity present in the water to form precipitate of Al(OH)3. The precipitate attracts the fine suspended particles present in the water to its surface causing them to clump together so that they can easily settle out of the water during the sedimentation process.
Chemical reaction -
Al2(SO4)3.18H2O+3Ca(HCO3)2 → 2Al(OH)3+3CaSO4+6CO2
CO2 is formed in the reaction which reacts with water to form carbonic acid (H2CO3) which will reduce the pH. Hence, when alum mixed with water then pH value of water decreases.