**Core Concept**
Bleaching powder, also known as calcium hypochlorite, is a widely used disinfectant and sanitizer in various settings, including water treatment and healthcare facilities. It contains a significant amount of available chlorine, making it effective against a broad spectrum of microorganisms.

**Why the Correct Answer is Right**
Bleaching powder is not a stable compound; it decomposes when exposed to air, moisture, and light, releasing chlorine gas. This decomposition process is accelerated in the presence of organic matter, making it essential to store bleaching powder in a cool, dry place. The instability of bleaching powder is a critical factor in its handling and storage, as it can lead to the release of toxic chlorine gas.

**Why Each Wrong Option is Incorrect**
**Option A:** This statement is true; bleaching powder indeed contains about 33% of available chlorine, which is responsible for its disinfectant properties.
**Option C:** This statement is also true; bleaching powder is a white amorphous compound, characterized by its powdery texture and white color.
**Option D:** This statement is true as well; bleaching powder is a strong bleaching agent, effective in removing stains and discolorations from surfaces.

**Clinical Pearl / High-Yield Fact**
It is essential to note that bleaching powder should be handled with caution, as it can release toxic chlorine gas when decomposed. This gas can cause respiratory irritation, damage to the eyes and skin, and even lead to respiratory failure in severe cases.

**✓ Correct Answer: B. It is stable compound**