Identify one federal agency involved in regulating water quality.

The Environmental Protection Agency (EPA) is the correct answer because it is the primary federal agency responsible for enforcing regulations related to water quality in the United States. The EPA's mission includes protecting human health and the environment by establishing standards and regulations under various environmental laws, such as the Clean Water Act. This agency implements and monitors programs that aim to prevent water pollution, ensure safe drinking water, and manage water resource planning.

The other agencies mentioned have different focuses; for instance, the Department of Natural Resources varies by state and generally deals with resource conservation rather than specific water quality regulations at the federal level. The National Parks Service primarily manages national parks and monuments, focusing on conservation and public enjoyment rather than regulatory efforts on water quality. The Department of Interior plays a broader role in managing natural resources and federal lands but does not specialize in water quality regulations like the EPA.