The OTC Bulletin Board (OTCBB) is BEST defined as?

The OTC Bulletin Board (OTCBB) is accurately defined as a quotation system for securities that are not listed on either the New York Stock Exchange (NYSE) or NASDAQ. This platform allows for the trading of over-the-counter (OTC) securities, which includes smaller companies and those that may not meet the requirements to be listed on major exchanges. The OTCBB provides a vital service by connecting buyers and sellers of these securities, offering increased visibility and access to these otherwise less-traded stocks.

The other options refer to different aspects of the financial markets. For instance, a platform for trading exchange-listed securities pertains to places like the NYSE or NASDAQ, which focus on stocks that meet certain criteria for listing. Likewise, a method for listing NYSE stocks is specific to that exchange and does not pertain to the OTC market. Lastly, a regulatory requirement for OTC markets would indicate obligations that trading activities must meet, but this would not serve as a direct definition of the OTCBB itself. Therefore, option C is the most fitting description of the OTC Bulletin Board's function within the trading ecosystem.