Which is not a type of Debenture :-

Debentures are classified into various types based on different criteria such as security, convertibility, redemption, and registration.

Analysis of each option:

Non-Convertible Debentures - These are debentures that cannot be converted into equity shares of the company. This is a valid type based on the criterion of convertibility.

Un-Secured Debentures - These are debentures that are not backed by any charge or security on the company's assets. Also known as naked debentures, this is a valid type based on the criterion of security.

Non-Transferable Debentures - Debentures are negotiable instruments by nature, meaning they can be freely transferred from one person to another. The very essence of a debenture is its transferability. There is no classification called "non-transferable debentures" in the standard types of debentures recognized under company law.

Irredeemable Debentures - These are debentures that have no fixed date of redemption and are redeemed only at the time of company's liquidation. Also called perpetual debentures, this is a valid type based on the criterion of redemption.

The standard classifications of debentures include:

• Based on Security: Secured/Unsecured
• Based on Convertibility: Convertible/Non-convertible
• Based on Redemption: Redeemable/Irredeemable
• Based on Registration: Registered/Bearer

Since debentures must be transferable instruments, Non-Transferable Debentures is not a recognized type of debenture.