March 23 (Reuters) - Robinhood Markets Inc, the online brokerage at the heart of the historic retail trading frenzy that gripped Wall Street this year, has confidentially submitted plans to regulators for a U.S. stock market listing, people familiar with the matter said. It was not immediately clear whether Robinhood has filed for a traditional initial public offering or a direct listing, but the company had considered going public through a direct listing in the weeks leading up to the filing, the sources added. A Robinhood spokeswoman declined to comment on the matter. Bloomberg News reported about the development earlier on Tuesday. (Reporting by Anirban Sen in Bengaluru; Editing by Shounak Dasgupta and Aditya Soni)