## Core Concept
Fresh bleaching powder is primarily composed of calcium hypochlorite, which is a strong disinfectant and bleaching agent. The chemical composition of bleaching powder can vary, but it mainly contains calcium hypochlorite (Ca(ClO)_2), calcium chloride (CaCl_2), and calcium hydroxide (Ca(OH)_2). The active ingredient responsible for its bleaching and disinfecting properties is calcium hypochlorite.

## Why the Correct Answer is Right
The correct answer, , indicates that fresh bleaching powder contains calcium hypochlorite (Ca(ClO)_2) as its main component. Calcium hypochlorite is a potent oxidizing agent that releases hypochlorous acid (HOCl) when it comes into contact with water. This reaction is responsible for its bleaching and antimicrobial properties: Ca(ClO)_2 + 2H_2O -> Ca(OH)_2 + 2HOCl. The hypochlorous acid is a strong disinfectant used for water treatment, surface cleaning, and as a bleach.

## Why Each Wrong Option is Incorrect
- **Option A:** - This option is incorrect because while bleaching powder may contain calcium chloride as a byproduct of its manufacturing process, it is not the primary component or the active ingredient responsible for its bleaching properties.
- **Option B:** - This option is incorrect because calcium hydroxide, although present, is not the primary active ingredient in fresh bleaching powder. It is more related to the alkaline nature of the mixture rather than its bleaching capability.
- **Option D:** - This option is incorrect because calcium carbonate is not a primary component of fresh bleaching powder. While it might be present in some form or as a contaminant, it does not represent the active bleaching agent.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the effectiveness of bleaching powder decreases over time due to its reaction with carbon dioxide in the air, which converts calcium hypochlorite into calcium carbonate, thus reducing its potency. Therefore, the composition and efficacy of bleaching powder can change significantly if it is not stored properly.

## Correct Answer Line
**Correct Answer: C. **