Option 1: Alum is not used as an analgesic, which is a substance used to relieve pain.
Option 2: Alum is not used as a fertilizer. Fertilizers are substances added to soil to improve plant growth, and alum does not have this property.
Option 3: Alum is not used as a disinfectant. Disinfectants are substances that kill or remove harmful microorganisms, and alum does not possess this property.
Option 4: The correct answer is 4. Alum is used as a purifier for water. Alum has the ability to remove impurities from water by causing the particles to settle at the bottom, thus making the water cleaner and safer to drink.
In conclusion, alum is not used as an analgesic, fertilizer, or disinfectant. Its primary use is as a purifier for water, making option 4 the correct answer.