Which of the following investments is a fixed income security?

A fixed income security is an investment that provides returns in the form of regular interest payments and the return of principal at maturity. This type of investment is characterized by predefined cash flows, which typically makes it a more stable and predictable option compared to other types of investments.

Corporate bonds are indeed a type of fixed income security. When an investor purchases a corporate bond, they are essentially lending money to a corporation for a specified period, and in return, the corporation agrees to pay interest at regular intervals as well as return the principal amount at the bond's maturity. This consistent return structure and relatively lower risk profile make corporate bonds a prime example of fixed income securities.

In contrast, common stocks represent ownership in a company and can yield returns through dividends and appreciation in share price, but they do not guarantee regular payments. Real estate investments can generate income through rent, but they are not categorized as fixed income since the returns can be variable and depend on market conditions. Index funds, while they may offer some level of income through dividends, are typically made up of stocks and are subject to market fluctuations, differing from the fixed income nature of bonds.