Over-collateralization is a credit enhancement technique where the value of the collateral (assets in the pool) is greater than the value of the issued securities. This technique provides a cushion for the investors in case of defaults or losses in the asset pool. The excess value of the assets over the securities serves as additional protection for the investors, increasing the creditworthiness of the securities. In the given scenario, where the nominal value of the assets in the pool is greater than the nominal value of the issued securities, over-collateralization is the most appropriate technique. It allows the issuer to take advantage of the excess value of the assets to enhance the credit of the securities.

Choice B is incorrect. Pool insurance is a form of external credit enhancement where an insurance company guarantees the payment of principal and interest to the bondholders in case of default by the issuer. However, in this scenario, since the nominal value of assets in the pool is greater than the nominal value of the issued securities, over-collateralization is more suitable.