Final answer:
Debentures are the types of bonds that are unsecured, relying on the issuer's creditworthiness rather than collateral.
Step-by-step explanation:
Among the given types of bonds, debentures are typically unsecured. A bond is a financial contract in which a borrower agrees to repay the borrowed amount along with interest over a specified period. Debentures are issued by companies and are not backed by collateral, meaning they are secured only by the general creditworthiness and reputation of the issuer. In contrast, mortgage bonds, guaranteed bonds, and collateral trust bonds are all secured by specific assets or collateral.