Which of the following pathogens are most commonly found naturally in the soil?

Clostridium tetani is the correct answer because it is a bacterium that is predominantly found in soil, dust, and animal feces. This anaerobic organism produces spores that can survive for long periods in the environment, especially in soil. When conditions are favorable, such as in an anaerobic environment (for instance, in a deep puncture wound), these spores can become active and produce the potent neurotoxin responsible for tetanus.

In contrast, Staphylococcus aureus is often associated with skin and mucosal surfaces and is more commonly found in human-associated environments rather than in soil. Escherichia coli, while it can be found in soil, is primarily associated with the intestines of warm-blooded animals and is more often related to fecal contamination. Salmonella enterica is typically found in the intestines of animals and can contaminate soil and water through fecal matter but is not naturally occurring in soil in the same way as Clostridium tetani. Thus, Clostridium tetani is distinctly recognized for its natural habitat in soil, making it the most appropriate choice among the options provided.