**Core Concept**
Bleaching powder, also known as calcium hypochlorite, is a disinfectant commonly used in water treatment and healthcare settings. It releases hypochlorous acid, which is responsible for its antimicrobial properties. The effectiveness of bleaching powder is often measured by the percentage of available chlorine it contains.

**Why the Correct Answer is Right**
The available chlorine in bleaching powder is typically measured by determining the amount of chlorine that can be released as hypochlorous acid when the powder is dissolved in water. This is usually expressed as a percentage of the total weight of the powder. The correct answer reflects the typical concentration of available chlorine in commercial bleaching powder products. The release of hypochlorous acid is facilitated by the reaction of calcium hypochlorite with water, which is catalyzed by the presence of hydrochloric acid.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it is lower than the typical concentration of available chlorine in bleaching powder.
**Option B:** This option is incorrect as it is higher than the typical concentration of available chlorine in bleaching powder.
**Option D:** This option is incorrect as it is not a recognized concentration of available chlorine in bleaching powder.

**Clinical Pearl / High-Yield Fact**
When using bleaching powder as a disinfectant, it is essential to ensure that the solution is prepared correctly, as incorrect concentrations can lead to reduced efficacy or even toxicity. A commonly recommended concentration for surface disinfection is 1-2% available chlorine.

**Correct Answer: C. 35%**