Which of the following contaminants does the U.S. EPA have enforceable standards for?

The correct choice highlights that the U.S. EPA has enforceable standards for a wide range of contaminants, specifically naming 90. These standards are part of the National Primary Drinking Water Regulations (NPDWR), which set maximum contaminant levels (MCLs) for various substances to protect public health. This includes various categories of contaminants such as chemicals, biological agents, and radiological elements, ensuring overall water safety for consumption.

Establishing these standards allows the EPA to regulate contaminants that have significant health implications, as well as to adapt standards to emerging contaminants over time, including those identified since certain regulations were first set in 2002. This capacity for regulatory evolution is crucial for effectively managing public health risks associated with drinking water.