Final answer:
Frank Riley has purchased a subordinated debenture.
Step-by-step explanation:
Frank Riley has purchased a subordinated debenture, which is a type of bond that is unsecured and is secondary to other unsecured bonds with respect to interest payments or repayment at maturity. This means that in case of default or bankruptcy, holders of other unsecured bonds will receive their payments before the holders of subordinated debentures.