All the options you provided can be seen as negative impacts of gold and silver mining in the West, but the most direct and broadly recognized negative effects would be:
A. Tent cities near the mines fostered criminals as long-term residents.
C. Mining camps were sources of many contagious diseases.
These two options highlight specific social and health-related issues that arose due to mining activities.
However, if you are looking for one that encapsulates a broader and long-term impact, B. Ghost towns were left behind after the mines stopped producing reflects the economic decline and abandonment that resulted from the boom-and-bust nature of mining towns.
Ultimately, if you have to choose one impact, it would be beneficial to identify which effect aligns best with the context of the discussion you are having, but all of these options certainly represent challenges encountered in the mining era.