**Core Concept**
Horrock's apparatus is a method used for water disinfection, specifically chlorination, to determine the required amount of bleaching powder. This method involves adding increasing amounts of bleaching powder to a series of cups containing water, observing the color change, and correlating it with the amount of bleaching powder required to achieve disinfection.

**Why the Correct Answer is Right**
The blue color from the 4th cup onwards indicates that the water has been adequately disinfected. According to Horrock's apparatus, this corresponds to a dosage of 0.4 mg/litre of chlorine. The bleaching powder required to disinfect the water can be calculated using the following formula: (Volume of water x Desired dose) / 33% (w/w concentration of bleaching powder). In this case, (75,000 litres x 0.4 mg/litre) / 0.33 = 90,909 mg or 90.91 g of bleaching powder.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided, but it would likely be an incorrect calculation or dosage.
* **Option B:** This option is not provided, but it would likely be an incorrect calculation or dosage.
* **Option C:** This option is not provided, but it would likely be an incorrect calculation or dosage.
* **Option D:** This option is not provided, but it would likely be an incorrect calculation or dosage.

**Clinical Pearl / High-Yield Fact**
When using Horrock's apparatus, it's essential to remember that the blue color from the 4th cup onwards indicates adequate disinfection, corresponding to a dosage of 0.4 mg/litre of chlorine. This method is a simple yet effective way to determine the required amount of bleaching powder for water disinfection.

**Correct Answer: B. 90.91 g of bleaching powder**