The debentures which are payable on the expiry of the specific period are

Debentures can be classified based on their redemption feature into two categories:

Redeemable Debentures are those which are repayable by the company after a specified period or on a particular date. The company has an obligation to return the principal amount to debenture holders at the end of the predetermined period.

Perpetual Debentures (also called Irredeemable Debentures) are those which do not have any fixed maturity date. These remain outstanding for the lifetime of the company and are repaid only at the time of liquidation.

The question specifically mentions debentures "payable on the expiry of the specific period" → this indicates a fixed maturity period → characteristic of Redeemable Debentures.

Zero coupon rate debentures and Specific coupon rate debentures are classifications based on the interest payment feature, not on redemption. Zero coupon debentures don't pay periodic interest but are issued at a discount, while specific coupon rate debentures pay interest at a predetermined rate. Neither of these terms addresses the redemption timeline.

Therefore, Redeemable Debentures is the correct answer.