CDOs (Collateralized Debt Obligations) became popular investment vehicles before the 2008 financial crisis. Regarding CDOs, which one of the following statements is most accurate?

a. They are traded on public stock exchange similar to stocks

b. They are considered a derivative as they are not directly secured by mortgages

c. They are fully guaranteed by the federal government

d. They are Mortgage Backed Bonds where the security is subprime mortgages