The correct answer is: Establish a VPC peering connection between VPC A and VPC B.

Explanation:
VPC peering enables private connectivity between two VPCs in the same or different AWS accounts. It allows traffic to flow between resources in both VPCs using private IP addresses without traversing the public internet. This is the most secure and efficient solution for enabling communication between an EC2 instance in one VPC and a database in another VPC within the same account. Once peered, you can configure route tables and security groups to allow only the necessary traffic (e.g., database port access).

Why the other options are incorrect:

Allowing all traffic from the EC2 instance's public IP is not secure. This exposes the database to external IP-based access and forces traffic to route over the public internet, which violates security best practices.

Making the DB instance publicly accessible and assigning a public IP significantly increases the attack surface. It is not necessary and is a poor design choice when both VPCs are in the same account and private connectivity options exist.

Creating a proxy EC2 instance in VPC B adds unnecessary complexity, introduces a single point of failure, and does not improve security. It also requires you to manage and maintain an additional EC2 instance.

Why this question is important: It tests your understanding of secure inter-VPC connectivity options and your ability to select the most appropriate networking solution that avoids the public internet.

Tip: When resources in different VPCs (but the same account or region) need private communication, VPC peering is usually the simplest and most secure choice unless there are overlapping CIDR blocks or more complex routing needs, in which case Transit Gateway may be better.

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