Final answer:
Endospores are not killed by low-level germicidal agents; they are resistant and require special methods for sterilization.
Step-by-step explanation:
The organism that is NOT killed by a germicidal agent from the options provided is D. endospores. Germicidal agents are classified by their effectiveness against different types of microorganisms. Low-level germicides are capable of killing vegetative bacterial cells and some enveloped viruses but are generally ineffective against endospores. Endospores are particularly resilient forms of some bacteria, including genera such as Bacillus and Clostridium. These endospores are known to survive harsh conditions and require special sterilization methods to be effectively killed.