**Core Concept**
Bacillus stearothermophilus, also known as Geobacillus stearothermophilus, is a thermophilic (heat-loving) bacterium that serves as a biological indicator in autoclave machines. This indicates its ability to withstand high temperatures, making it a reliable choice for quality control.

**Why the Correct Answer is Right**
Bacillus stearothermophilus is used as a biological indicator due to its exceptional heat resistance. This bacterium forms highly resistant spores that can survive temperatures above 121°C (250°F) for extended periods. The autoclave machine's ability to kill Bacillus stearothermophilus spores indicates its effectiveness in sterilizing equipment and supplies. The presence of these spores also helps in verifying the proper functioning of the autoclave.

**Why Each Wrong Option is Incorrect**
**Option A:** Clostridium tetani is a pathogenic bacterium responsible for tetanus, which is not typically used as a biological indicator in autoclave machines.
**Option C:** Bacillus pumilus is a spore-forming bacterium but is not as heat-resistant as Bacillus stearothermophilus and therefore not suitable for quality control in autoclaves.
**Option D:** Bacillus subtilis is another spore-forming bacterium, but it is not as thermophilic as Bacillus stearothermophilus and thus not the preferred choice for autoclave validation.

**Clinical Pearl / High-Yield Fact**
The use of Bacillus stearothermophilus as a biological indicator in autoclave machines ensures the effective sterilization of medical equipment, which is crucial in preventing hospital-acquired infections. This emphasizes the importance of regular autoclave maintenance and quality control.

**✓ Correct Answer: B. Bacillus stearothermophilus**