In the case file the organism responsible for the patient's death is a normal member of the human microbiota. how do you explain it cause the death of the patient

The patient was immunocompromised or the microbiota moved to an unprotected sites in the patient.

Explanation:

The normal member of the human microbiota can cause disease in immunocompromised individuals or when the microbiota move to an unprotected sites in the body of an individual.

Immunocompromised individuals are those whose immune systems are not fully functional due to reasons such as genetic defects, taking of immunnosuppressive drugs, or infection by immune-targeting pathogens.  

Normal microbiota are known to be opportunistic pathogens that are capable of causing infections when a host becomes immunocompromised or when they moved from their regular habitats to an unprotected site within the body.

For example, Staphylococcus epididermis which happens to be one of the normal microbiota of the skin's epidermis might cause an infection if the surface of the skin breaks and the organism moves to the deeper lying tissues.

Hence, it could be that the patient was immunocompromised or the mirobiota moved to an unprotected sites within the body of the patient.

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."