A. Create a peering connection between the VPCs. Add a route table entry for the peering connection in both VPCs. Configure an inbound rule for the ElastiCache cluster’s security group to allow inbound connection from the application’s security group. Creating a peering connection between the VPCs allows the application's EC2 instances to communicate with the ElastiCache cluster directly and efficiently. This is the most cost-effective solution as it does not involve creating additional resources such as a Transit VPC, and it does not incur additional costs for traffic passing through the Transit VPC. Additionally, it is also more secure as it allows you to configure a more restrictive security group rule to allow inbound connection from only the application's security group.

A. Create a peering connection between the VPCs. Add a route table entry for the peering connection in both VPCs. Configure an inbound rule for the ElastiCache cluster’s security group to allow inbound connection from the application’s security group. Creating a peering connection between the VPCs allows the application's EC2 instances to communicate with the ElastiCache cluster directly and efficiently. This is the most cost-effective solution as it does not involve creating additional resources such as a Transit VPC, and it does not incur additional costs for traffic passing through the Transit VPC. Additionally, it is also more secure as it allows you to configure a more restrictive security group rule to allow inbound connection from only the application's security group.

Create a peering connection between the VPCs. Add a route table entry for the peering connection in both VPCs. Configure an inbound rule for the ElastiCache cluster’s security group to allow inbound connection from the application’s security group.

Create a VPC peering connection between the Cache VPC and App VPC. This allows private IP connectivity between the VPCs. Add route table entries in each VPC to route traffic destined to the other VPC via the peering connection. This enables network routing. Configure security groups to allow inbound connections from the application instances to the ElastiCache cluster.

Creating a peering connection between the VPCs is a cost-effective way to establish connectivity. By adding a route table entry for the peering connection in both VPCs, traffic can flow between them. Configuring an inbound rule in the ElastiCache cluster's security group allows inbound connections from the application's security group, enabling access to the ElastiCache cluster from the EC2 instances in the App VPC. Option B suggests creating a Transit VPC, which adds unnecessary complexity and cost for this scenario. Option C suggests configuring an inbound rule for the peering connection's security group, which is not necessary as the security group for the ElastiCache cluster should be used to control inbound connections. Option D suggests configuring an inbound rule for the Transit VPC's security group, which is not needed in this case and adds unnecessary complexity. Therefore, option A is the most cost-effective solution to provide the application's EC2 instances with access to the ElastiCache cluster.

A is correct, 1. VPC transit is used for more complex architecture and can do VPCs to VPCs connectivity. But for simple VPC 2 VPC can use peer connection. 2.To enable private IPv4 traffic between instances in peered VPCs, you must add a route to the route tables associated with the subnets for both instances. So base on 1, B and D are out, base on 2 C is out

Why not C ? any explanation?

Cost Effectively!