Which of the following organisms are most resistant to unfavorable environmental conditions and indicate past or possibly intermittent pollution?

Clostridium sporulates is the correct answer because this organism forms spores, which are highly resistant to unfavorable environmental conditions such as extreme temperatures, dehydration, and exposure to toxic substances. This remarkable resilience allows Clostridium spores to survive in environments where other microorganisms cannot, making them excellent indicators of past or intermittent pollution. Their ability to persist in soil and sediment for long periods signifies that a contaminated site may have experienced pollution events, even if current testing does not show the presence of other more sensitive indicators.

In contrast, while fecal streptococci, Escherichia coli, and Aerobacter aerogenes may be associated with fecal pollution and are relevant for water quality monitoring, they do not possess the same level of resistance to environmental stressors as Clostridium sporulates. These organisms typically indicate recent contamination rather than long-term survival in the environment, which makes them less suitable for indicating past pollution events.