Perpetual Debentures are those debentures that have no fixed date of redemption. The company is not obligated to repay the principal amount during its lifetime and only pays interest periodically to the debenture holders.
These are also known as Irredeemable Debentures because they cannot be redeemed or repaid by the company (except in case of liquidation).
The other options represent different classifications of debentures:
Non-Convertible Debentures → Cannot be converted into equity shares, but they do have a maturity date for redemption
Unsecured Debentures → Not backed by any specific asset or collateral of the company
Registered Debentures → Registered in the books of the company with details of debenture holders recorded
The distinguishing feature of perpetual debentures is the absence of a redemption date, which is why they are termed as irredeemable.