**Core Concept**
The question tests the understanding of **disinfection** and **bleaching power** calculation, specifically in the context of **Horrock's apparatus**, which is used to determine the amount of available chlorine in a bleaching solution. The **starch-iodine indicator** is used to detect the presence of available chlorine.

**Why the Correct Answer is Right**
Since the second cup of Horrock's apparatus shows blue color after 3 drops of starch-iodine indicator, it indicates that the available chlorine in the solution is sufficient to disinfect. The amount of available chlorine required to disinfect is directly proportional to the volume of water being disinfected. To calculate the dose of bleaching power required, one needs to consider the volume of water (910 L) and the amount of available chlorine required.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not take into account the correct proportion of available chlorine required for disinfection.
**Option B:** This option is incorrect because it underestimates the amount of bleaching power required for the given volume of water.
**Option D:** This option is incorrect because it overestimates the amount of bleaching power required, which could lead to excessive chlorine residual.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the amount of available chlorine required for disinfection varies depending on the volume of water, contact time, and the presence of organic matter. Always follow standard guidelines for disinfection to ensure effective and safe use of bleaching power.

**Correct Answer:** D. 182 grams