Final answer:
B) Bacillus subtilis spores are used to test steam sterilizers due to their heat resistance, serving as a biological indicator of effective sterilization.
Step-by-step explanation:
The spore-forming organism commonly used to test the proper functioning of a steam sterilizer is B) Bacillus subtilis. This particular microorganism is used as a biological indicator because its spores are highly resistant to heat and other sterilization methods. In the context of verifying sterilization processes, spores of Bacillus subtilis are used due to their ability to withstand extreme conditions, providing assurance that if these spores are inactivated, other potential pathogens would also be effectively killed during the sterilization process.