To enhance the security of the notification controller exposed for marking orders as paid, the recommended approach is to add an order-specific token to the callback URL (Answer C). This token should be verified against a stored value on the order to ensure the authenticity and integrity of the payment notification. This method prevents unauthorized use of the controller, as only callbacks containing the correct, order-specific token would be allowed to mark orders as paid. This strategy is essential in maintaining secure and reliable transaction processes within Salesforce B2C Commerce.