FINRA's core function is regulating brokerage firms and registered representatives to ensure fair and honest markets. It establishes and enforces rules governing trading practices, licensing, disclosure, and ethical conduct. FINRA also conducts examinations, investigates misconduct, and administers arbitration and mediation between investors and brokers. Unlike the Federal Reserve or FDIC, FINRA does not manage monetary policy or insure deposits. Financial management and regulatory texts consistently describe FINRA as a critical component of U.S. securities market oversight. Option D correctly identifies its primary role.