**Core Concept**
The efficiency of sterilization in an autoclave is crucial to prevent the transmission of infectious agents in medical settings. Autoclaves use high-pressure steam to achieve sterilization, and a biological indicator is necessary to validate their effectiveness.

**Why the Correct Answer is Right**
The correct answer, Geobacillus stearothermophilus (formerly known as Bacillus stearothermophilus), is a thermophilic bacterium that forms highly resistant spores. These spores are used as a biological indicator to test the efficiency of autoclave sterilization. When exposed to high-pressure steam, the spores will either be killed or not, depending on the effectiveness of the sterilization process. The presence of live spores after autoclaving indicates inadequate sterilization.

**Why Each Wrong Option is Incorrect**
* **Option A:** Bacillus subtilis is another bacterium that can form spores, but it is not typically used as a biological indicator for autoclave sterilization. While it can be used for other purposes, it is not the preferred choice for this specific application.
* **Option B:** No information is provided for this option, making it impossible to evaluate its correctness.
* **Option D:** No information is provided for this option, making it impossible to evaluate its correctness.

**Clinical Pearl / High-Yield Fact**
When validating the efficiency of an autoclave, it's essential to use a biological indicator that can withstand the high temperatures and pressures involved in the sterilization process. Geobacillus stearothermophilus spores are the gold standard for this purpose due to their exceptional heat resistance.

**Correct Answer:** C. Geobacillus stearothermophilus (formerly known as Bacillus stearothermophilus).