Perpetual Debentures are debentures issued without any fixed maturity date. The company is not required to repay the principal amount at any specific time and only pays interest to debenture holders periodically throughout the company's existence.
These debentures are also known as Irredeemable Debentures because they cannot be redeemed or repaid during the lifetime of the company. The term "irredeemable" emphasizes that there is no redemption (repayment) obligation.
The other options represent different classifications of debentures:
Non-Convertible Debentures are classified based on convertibility (cannot be converted into shares)
Unsecured Debentures are classified based on security (not backed by any specific asset)
Registered Debentures are classified based on transferability (holder's name recorded in company's register)
Since perpetual debentures have no maturity period and remain outstanding indefinitely, they are correctly termed as Irredeemable Debentures.