Which organization is responsible for enforcing laws related to controlled substances in the United States?

The correct answer to the question is the Drug Enforcement Administration (DEA). This agency is specifically tasked with enforcing the controlled substances laws and regulations of the United States. The DEA's responsibilities include overseeing the legal manufacture and distribution of controlled substances, ensuring compliance with the Controlled Substances Act, and combating drug trafficking and abuse.

The DEA plays a critical role in preventing illegal drug use by enforcing penalties for violations, regulating schedules of substances, and monitoring compliance among manufacturers and practitioners who handle drugs.

Other organizations listed, such as the Environmental Protection Agency, National Institutes of Health, and Food and Drug Administration, have important regulatory functions but are not directly responsible for the enforcement of controlled substances laws. The EPA focuses on environmental protection, the NIH conducts medical research, and the FDA regulates food and drug safety but does not enforce controlled substance laws. Hence, these choices do not align with the specific role that the DEA holds in the context of controlled substances.