Final answer:
Bacillus atrophaeus is the microorganism used to biologically monitor the dry heat sterilizer, thanks to its heat-resistant endospores.
Step-by-step explanation:
The microorganism used to biologically monitor the dry heat sterilizer is Bacillus atrophaeus. This particular species is preferred because it produces endospores that are very resistant to heat and are therefore a good indicator of sterilization effectiveness. Bacillus subtilis is also used in some cases, and while Bacillus stearothermophilus is used for steam sterilization monitoring, it is not typically used for dry heat sterilizers. Clostridium difficile, although an endospore-forming bacterium, is not used for monitoring sterilization processes.