The correct answer is to configure the application using App Engine Standard environment with Cloud VPN to connect to an on-premise database. Cloud VPN is necessary to connect a VPC to an on-premise network, and since there's no requirement to run Docker containers, App Engine Standard, which supports Java runtime environments, is the optimal choice to minimize effort. Cloud Router alone is insufficient for connecting a VPC to an on-premise network as it only enables dynamic route exchange between networks using BGP. References: Cloud VPN Overview, Cloud Router.