You have been asked to leverage Amazon VPC EC2 and SQS to implement an application that processes millions of messages per second from a message queue. You want to ensure that your application in EC2 instances has sufficient bandwidth when the SQS queue grows. The instances also need to access the internet. Which option will provide the most scalable solution for the application?

Answer options:

A.Ensure the application instances are properly configured with an Elastic Load Balancer.
B.Ensure the application instances are launched in private subnets with the EBS-optimized option enabled.
C.Ensure the application instances are launched in private subnets with the associate-public-IP-address=true option enabled. Remove any NAT instance from the public subnet, if any.
D.Ensure the application instances are launched in public subnets with an Auto Scaling group and Auto Scaling triggers are configured to watch the SQS queue size.

Answer – D
The Amazon EC2 instance requires access to the Internet. Hence, it should be in a public subnet or be in a private subnet with a NAT instance/gateway in the public subnet.
Option A is incorrect because ELB does not ensure scalability.
Option B is incorrect because (a) the EBS-optimized option will not contribute to scalability, and (b) there should be a NAT instance/gateway in the public subnet of the VPC for accessing SQS.
Option C is incorrect because if you remove the NAT instance, the EC2 instance cannot access the SQS service.
Option D is CORRECT because (a) it uses Auto Scaling for ensuring scalability of the application, and (b) it has instances in the public subnet so they can access the SQS service. 
For more information on SQS, please visit the below URL-
https://aws.amazon.com/sqs/faqs/