Final answer:
The correct answer to the student's question is a) Listeria monocytogenes, as it is a regular non-spore-forming gram-positive bacillus pathogen that causes listeriosis.
Step-by-step explanation:
The student is asking which of the given options is a regular non-spore-forming gram-positive bacillus pathogen. Among the options, Listeria monocytogenes is a non-spore-forming, nonencapsulated, gram-positive rod and is known to cause listeriosis. This is unlike Bacillus anthracis and Bacillus cereus, both of which are endospore-forming pathogens, and Clostridium perfringens, which also forms endospores and is not a bacillus but rather a rod-shaped bacterium. Therefore, the correct answer is a) Listeria monocytogenes.