Filtration serves as the last barrier between the public and which microorganisms?

Filtration is critical in water treatment, acting as a crucial barrier to ensure safe drinking water. The correct choice highlights Giardia and Cryptosporidium, two protozoan parasites that are resilient and can survive in various water sources, including untreated or inadequately treated water. These microorganisms are particularly concerning due to their ability to cause gastrointestinal illnesses in humans.

During the filtration process, the physical barriers present in water treatment facilities capture these protozoa, preventing them from entering the water supply. This is vital because even a small number of Giardia or Cryptosporidium can lead to outbreaks of disease. While other options mention different types of microorganisms or contaminants, they do not encapsulate the primary focus of filtration in safeguarding against the most concerning pathogens that pose direct health risks.

In conclusion, the emphasis on Giardia and Cryptosporidium underlines the specific threats that filtration addresses, making it the correct answer in the context of ensuring safe drinking water through effective contaminant removal.