Answer:
The answer is D. an unsecured long-term debt
Step-by-step explanation:
Debenture is a bond because it is a long-term loan and debenture is not secured (unsecured). It is unsecured in the sense that there is no collaterals but relies on the creditworthiness of the issuers.
Option A is incorrect becausebond is usually not bonds below Investment grade.
Option B is incorrect because debenture are nor secured by any properties.
Option C is incorrect because debenture makes coupon interest payment.