Final answer:
The trade between Shareholder A and Shareholder B occurred in the Secondary market.
Step-by-step explanation:
When Shareholder A sold shares of Maplewood Cabinets stock to Shareholder B, this trade occurred in the Secondary market. The secondary market is where existing securities, such as stocks, are bought and sold among investors. The New York Stock Exchange (NYSE) is an example of a secondary market where shares of listed companies can be traded.