Explanation
Option A is correct because enabling the config flag under deployment/blue_green/enabled is one of the steps to ensure zero downtime during the deployment process of Magento 2 on-premises. This flag enables the blue-green deployment feature, which allows deploying a new version of the Magento application to a separate environment (blue) without affecting the current live environment (green). Once the new version is ready, the traffic can be switched from green to blue with minimal or no downtime1.
Option C is correct because running bin/magento setup:upgrade --keep-generated is another step to ensure zero downtime during the deployment process of Magento 2 on-premises. This command updates the database schema and data without deleting the generated code and static view files. This way, the Magento application can still serve requests from the cache while the database is being upgraded2.
Option B is incorrect because running bin/magento setup:upgrade --dry-run=true does not upgrade the database, but only checks if there are any errors or conflicts in the database schema or data. This command can be used for testing purposes, but it does not affect the deployment process or the downtime3.
Option D is incorrect because there is no such option as --convert-old-scripts-true for the bin/magento setup:upgrade command. This option does not exist in Magento 2 and does not have any effect on the deployment process or the downtime.
Option E is incorrect because there is no such config flag as developer/zero_down_time/enabled in Magento 2. This flag does not exist in Magento 2 and does not have any effect on the deployment process or the downtime.
References:
1: Blue-green deployment | Adobe Commerce Developer Guide
2: Deploy Magento to production | Adobe Commerce Developer Guide
3: Command-line installation options | Adobe Commerce Developer Guide