Question

read this us policy on clean water.
the clean water act (cwa) establishes the basic structure for regulating discharges of pollutants into the waters of the united states and regulating quality standards for surface water.
—summary of the clean water act, environmental protection agency
how can the clean water act best promote health
by monitoring a toxic spill that affects a drinking water supply
by providing pamphlets related to the quality of drinking water
by carrying out research about the effects of large discharges
by controlling the amount of water people use in their homes

Explanation:

The Clean Water Act focuses on regulating pollutant discharges. Monitoring a toxic spill affecting drinking water supply directly relates to its core function of regulating water - related pollutants. Providing pamphlets (more of an informational but not regulatory action), carrying out research (not the main regulatory action for immediate health promotion in the context of the act's described role), and controlling home water use (not the act's main focus as per the summary) are less relevant.

Answer:

by monitoring a toxic spill that affects a drinking water supply