You are deploying an application on App Engine that needs to integrate with an on-premises database. For security purposes, your on-premises database must not be accessible through the public internet. What should you do?

The correct answer is to deploy your application on App Engine flexible environment and use Cloud VPN. App Engine standard environment instances cannot connect to resources in a VPC network, which is required for Cloud VPN or Cloud Interconnect. The flexible environment, however, runs within a Google Cloud project's VPC network, allowing it to establish secure, private connections to on-premises resources via Cloud VPN or Cloud Interconnect. App Engine firewall rules only control inbound traffic to your App Engine application, not outbound connections to an on-premises database. Therefore, Cloud VPN is necessary to create a secure tunnel for private communication between your App Engine flexible environment application and the on-premises database.