When securities are sold by one investor and purchased by another investor, the transactions occur in which of the following markets?

The correct choice is the secondary market because this is the platform where previously issued securities are traded between investors. When one investor sells a security they own, and another investor purchases it, this transaction does not involve the issuing company; hence, it takes place in the secondary market.

In contrast, the primary market is concerned with the issuance of new securities directly from issuers to investors. Transactions in the primary market occur when companies go public or issue additional stocks or bonds. The money market pertains to the trading of short-term debt securities and is distinctly different from transactions involving ownership of stocks, which are usually longer-term investments. The term "negotiable" relates more to the characteristics of certain financial instruments that can be transferred or sold, but it does not represent a specific market for trading securities.

Thus, since the question specifies transactions between investors with no new issuance involved, the secondary market is indeed the appropriate context.