He 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.

A cholera epidemic.

Explanation:

The cause of cholera is scientifically proven or believed to be from contamination of foods or drinkable water in which humans later take into their body system. This is often common where there is an inefficient water supply system.

Hence, in this case, the most serious effect of the poor sanitation and contaminated water supplies in a tenement ( a typical large building, which is occupied by quite a several people) is "a cholera epidemic."

c) they were beaten and fired from their jobs.