Final answer:
The true statement is that all stock trades between existing shareholders are secondary market transactions because secondary markets are where existing securities are traded among investors.
Step-by-step explanation:
The correct statement among the options provided is C. All stock trades between existing shareholders are secondary market transactions. Secondary markets are where existing securities, such as stocks and bonds, are bought and sold among investors or traders. These markets provide liquidity, allowing investors to easily sell securities. Secondary markets exist in various forms, with some being dealer markets, where dealers buy and sell securities from their own inventory, and others being broker markets, where brokers facilitate trades between buyers and sellers. Not all secondary markets are strictly dealer markets or broker markets, so statements A and B are false. Statement D is also false because not all stock transactions occur in the secondary market; initial public offerings (IPOs), for example, are part of the primary market.