Bleaching powder is commonly used:

Correct Option: To disinfect water

Explanation:
Bleaching powder, chemically known as calcium hypochlorite with the formula ${\mathrm{CaOCl}}_2$, is widely used as a disinfectant and germicide, especially for water treatment.
When dissolved in water, bleaching powder releases chlorine gas, which effectively kills harmful microorganisms, bacteria, and germs, making the water safe for drinking.

Let's briefly examine the choices:
• As a sweetener: Sweeteners are substances like sucrose, fructose, or artificial alternatives (e.g., aspartame). Bleaching powder is toxic if consumed.
• To make soap: Soap making (saponification) requires fats or oils reacting with a strong base like sodium hydroxide ($\mathrm{NaOH}$).
• To disinfect water: This is the correct application of bleaching powder due to its strong oxidizing and antibacterial properties.
• As a fuel: Bleaching powder is non-combustible and cannot be used as a fuel source.