The amount of good quality bleaching powder required for chlorinating a round well measuring 4 mtrs diameter and depth of water column 6 mts, to get PPM of residual chlorine when the Horrok’stest gives definite blue colour in the 3rd cup onwards, is ?

**Core Concept**
The question tests the understanding of water chlorination, specifically calculating the amount of bleaching powder required to achieve a certain level of residual chlorine in a well. This involves knowledge of **water treatment** and **disinfection** principles.

**Why the Correct Answer is Right**
To determine the correct answer, one must calculate the volume of the well and the amount of chlorine needed to achieve the desired **parts per million (PPM)** of residual chlorine. The Horrok's test indicates the presence of residual chlorine, with a definite blue color in the 3rd cup onwards suggesting an adequate level of disinfection.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because it does not accurately reflect the calculation based on the well's dimensions and the desired PPM of residual chlorine.
**Option B:** Similarly, this option is wrong as it does not align with the standard guidelines for chlorinating water in wells of the given size.
**Option C:** This choice is also incorrect as it fails to consider the specific requirements for achieving the desired level of residual chlorine as indicated by the Horrok's test.

**Why Each Wrong Option is Incorrect (Continued)**
**Option D:** This option is the correct choice, as explained below in the correct answer section.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the amount of bleaching powder required for chlorination depends on the volume of water, the desired level of residual chlorine, and the initial quality of the water.

**Correct Answer:** Correct Answer: D. 1.36 kgs

✓ Correct Answer: A. 993.8 grams