The answer is D. Collateral trust bond.
A collateral trust bond is a type of bond where the corporation pledges securities which it owns such as the stock or bonds of one of its subsidiaries. The pledged securities are held by a trustee, who is responsible for ensuring that the bondholders are paid if the corporation defaults on its debt.
Mortgage bonds are secured by a mortgage on real property. Joint bonds are issued by two or more corporations, and each corporation is jointly and severally liable for the debt. Security bonds are secured by a pledge of the corporation’s general assets.