Which organization is responsible for tax collection and tax law enforcement in the United States?

The Internal Revenue Service (IRS) is the organization tasked with tax collection and the enforcement of tax laws in the United States. This federal agency operates under the Department of Treasury and is specifically responsible for managing the nation's tax system, ensuring compliance with tax regulations, and administering the Internal Revenue Code. The IRS handles a wide range of responsibilities, including processing tax returns, conducting audits, collecting taxes, and enforcing penalties for tax violations.

The other organizations listed have different focuses: the Department of Treasury oversees financial and monetary policy but does not directly engage in tax collection; the FBI is primarily concerned with law enforcement and the investigation of federal crimes; and the Social Security Administration manages social security programs, including retirement and disability benefits, but does not handle tax collection duties. Thus, the IRS is uniquely positioned to handle all matters related to federal taxes, making it the correct answer to the question.