A. Create a VPC with one public and one private subnet. Launch an application instance in the public subnet while RDS is launched in the private subnet.
B. Setup a public and two private subnets in different AZs within a VPC and create a subnet group. Launch RDS with that subnet group.
C. Create a network interface and attach two subnets to it. Attach that network interface with RDS while launching a DB instance.
D. Create two separate VPCs and launch a Web app in one VPC and RDS in a separate VPC and connect them with VPC peering.