**Core Concept**
Autoclaving, hydroclaving, and microwaving are methods used to sterilize medical equipment and supplies. To ensure the effectiveness of these methods, a biological indicator or a chemical indicator is used to verify the sufficiency of the sterilization process.

**Why the Correct Answer is Right**
The correct answer is **B. Geobacillus stearothermophilus**. This bacterium is commonly used as a biological indicator for autoclaving, hydroclaving, and other forms of steam sterilization. Its spores are highly resistant to heat and can withstand the high temperatures and pressures generated during these processes. If the spores are inactivated, it indicates that the sterilization process was effective.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Bacillus subtilis** is used as a biological indicator for dry heat sterilization, not for autoclaving or hydroclaving.
* **Option C:** **Escherichia coli** is a common laboratory bacterium, but it is not typically used as a biological indicator for sterilization processes.

**Clinical Pearl / High-Yield Fact**
When using biological indicators for sterilization, it's essential to follow the manufacturer's instructions and guidelines for the specific type of indicator being used. This ensures accurate results and helps prevent equipment and supplies from being contaminated.

**Correct Answer:** B. Geobacillus stearothermophilus