**Core Concept**
Dry heat sterilization is a method used to eliminate microorganisms by exposing them to high temperatures. Bacterial indicators are used to validate the effectiveness of the sterilization process.

**Why the Correct Answer is Right**
The correct answer is Geobacillus stearothermophilus (also known as Bacillus stearothermophilus). This bacterium is commonly used as a biological indicator for dry heat sterilization due to its high heat resistance and spore-forming ability. When exposed to high temperatures, G. stearothermophilus spores will not germinate or die, indicating that the sterilization process has been effective. This is because the bacterium's spores are highly resistant to heat, making them an ideal indicator for validating dry heat sterilization.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because Bacillus subtilis var. niger is actually used as a biological indicator for moist heat sterilization, not dry heat sterilization.
* **Option B:** This option is incorrect because Clostridium sporogenes is a bacterium that can be used as a biological indicator for sterilization, but it is not specific to dry heat sterilization.
* **Option D:** This option is incorrect because the correct answer is Geobacillus stearothermophilus, not Bacillus stearothermophilus.

**Clinical Pearl / High-Yield Fact**
When using bacterial indicators for sterilization, it is essential to select the correct indicator organism for the specific sterilization method being used. Failure to do so can result in inaccurate validation of the sterilization process.

**Correct Answer: C. Geobacillus stearothermophilus**