Debentures are a type of long-term debt instrument issued by a company or government entity to raise funds. They are essentially loan agreements between the issuer and the debenture holders, who lend money to the issuer in exchange for regular interest payments and the repayment of the principal amount at maturity.

The given options are as follows:

a) They can be issued for cash: This statement is true. Debentures can be issued in exchange for cash. When a company needs to raise funds, it may issue debentures to investors who provide cash in return. The company can then use this cash for various purposes, such as financing capital expenditures, expanding operations, or paying off existing debt.

b) They can be issued for consideration other than cash: This statement is also true. Debentures can be issued for consideration other than cash. This means that instead of receiving cash, the debenture holders may receive other assets, such as shares of stock, property, or even services. This allows the issuer to raise funds while avoiding the immediate cash outflow.

c) They cannot be issued as collateral security: This statement is false. Debentures can be issued as collateral security. Collateral security refers to assets or property that are pledged by the issuer as a guarantee for the repayment of the debentures. In case of default, the debenture holders have a claim on the collateral assets to recover their investment. This provides an additional layer of security for the debenture holders.

d) Both (a) and (b) above: This statement is true. Both options (a) and (b) are correct, as explained above. Debentures can be issued for cash as well as for consideration other than cash.

In conclusion, the correct answer is option 'D'. Debentures can be issued for cash or consideration other than cash, and they can also be issued as collateral security.