The defining feature of a financial product governed by the Securities Investor Protection Corporation is:

The defining feature of a financial product governed by the Securities Investor Protection Corporation (SIPC) is protection against a broker's default. The SIPC is a U.S. government-backed organization that protects customers of brokerage firms in the event that the firm fails. If a broker goes bankrupt or experiences fraud, the SIPC provides coverage to customers to help recover their securities and cash, up to certain limits. This is designed to instill confidence in the securities market by ensuring that individual investors are protected from losing their investments due to a broker's inability to manage their accounts properly.

In contrast, options related to tax advantages, market stability, and high return rates do not specifically encapsulate the essence of what the SIPC does. While financial products may offer tax benefits or promise market returns, these aspects are not relevant to the protective measures established by the SIPC in the context of broker insolvency. The uniqueness of the SIPC lies in its specific focus on safeguarding investors from the risk of a brokerage firm's failure, which is fundamentally different from the other characteristics mentioned.