This solution provides the highest level of security by implementing a defense-in-depth strategy. Placing the Amazon RDS cluster in private subnets ensures it is not directly accessible from the public internet, which is a fundamental security best practice for databases. An AWS Site-to-Site VPN establishes a secure, encrypted IPsec tunnel between the company's on-premises office and the AWS VPC. This allows the desktop clients to communicate with the RDS cluster over a private, encrypted connection as if it were on the local network, without exposing any resources to the public internet. This combination of network isolation (private subnets) and encrypted transit (VPN) represents the most secure architecture for the given requirements.

A. Placing the RDS cluster in public subnets exposes it to the internet, which is a significant and unnecessary security risk, even with a VPN.

C. This option is incomplete. Security groups alone cannot establish connectivity from an on-premises network to private subnets within a VPC; a VPN or AWS Direct Connect is required.

D. This is the least secure option. It places the database in public subnets and relies only on security groups for network filtering, creating a direct attack surface on the internet.

1. AWS Documentation - Amazon RDS User Guide: Under "Security in Amazon RDS

" the section "Using Amazon RDS with Amazon VPC" states

"A common strategy is to have a public subnet with a web server that has access to the internet

and a private subnet with no internet access that contains your DB instances... We recommend that you run your DB instance in a private subnet." This supports placing RDS in private subnets for security.

2. AWS Documentation - AWS Site-to-Site VPN User Guide: The "What is AWS Site-to-Site VPN?" section explains

"By default

instances that you launch into an Amazon VPC can't communicate with your own (remote) network. You can enable access to your remote network from your VPC by creating an AWS Site-to-Site VPN (Site-to-Site VPN) connection..." This confirms VPN is the correct tool for secure on-premises to VPC connectivity.

3. AWS Whitepapers - "Amazon Virtual Private Cloud Connectivity Options": This whitepaper details connectivity patterns. In the section "AWS Managed VPN

" it describes the service as a way to "establish an encrypted connection between your on-premises network and your Amazon VPCs over the internet." This validates the use of a VPN for secure

private communication.

4. AWS Documentation - Amazon VPC User Guide: The "Subnets for your VPC" section clarifies the distinction: "Resources in a private subnet can't be accessed from the internet." This reinforces why databases should be placed in private subnets to meet the "MOST securely" requirement.