**Core Concept**
The question requires the application of Horrock's apparatus to calculate the amount of bleaching powder needed to disinfect a large volume of water. Horrock's apparatus is a method used to determine the residual chlorine in water, which is essential for water disinfection.

**Why the Correct Answer is Right**
In Horrock's apparatus, four, five, and six cups show distinct coloration, indicating a certain amount of residual chlorine. This corresponds to the amount of chlorine required to disinfect 1,000 liters of water. To calculate the amount of bleaching powder required for 455 liters of water, we need to proportionally adjust the amount of chlorine. Since four, five, and six cups show distinct coloration, we can assume that the amount of chlorine required is proportional to the volume of water being disinfected.

**Why Each Wrong Option is Incorrect**
**Option A:** This option may be tempting, but without knowing the exact amount of chlorine required for the given volume of water, it's difficult to determine the correct amount of bleaching powder needed.

**Option B:** This option may be incorrect because it doesn't take into account the proportionality of chlorine to the volume of water being disinfected.

**Option C:** This option may be incorrect because it doesn't provide a clear calculation or proportionality to the volume of water being disinfected.

**Clinical Pearl / High-Yield Fact**
To calculate the amount of bleaching powder required for water disinfection using Horrock's apparatus, remember that four, five, and six cups show distinct coloration, indicating a certain amount of residual chlorine. The amount of chlorine required is proportional to the volume of water being disinfected.

**Correct Answer:** 3.