The most serious effect of the poor sanitation and contaminated water supplies in tenements was: filthy living conditions. a cholera epidemic. a public-health problem. immigrants were unable work.

All are impacts of the poor water conditions but cholera outbreaks were the worst. 

Cholera is the result of contaminated water due to improper sewage systems. Human and animal waste often came into contact with drinking water causing sickness and death among those living in these poor conditions. 

B. A cholera Epidemic

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Cis the correct answer