Which AWS services would be best to use to accomplish this?

You have been given a scope to deploy some AWS infrastructure for a large organization. The requirements are that you will have a lot of EC2 instances but may need to add more when the average utilization of your Amazon EC2 fleet is high and conversely remove them when CPU utilization is low.

Which AWS services would be best to use to accomplish this?
A . Auto Scaling, Amazon CloudWatch and AWS Elastic Beanstalk
B . Auto Scaling, Amazon CloudWatch and Elastic Load Balancing.
C . Amazon CloudFront, Amazon CloudWatch and Elastic Load Balancing.
D . AWS Elastic Beanstalk, Amazon CloudWatch and Elastic Load Balancing.

Answer: B

Explanation:

Auto Scaling enables you to follow the demand curve for your applications closely, reducing the need to manually provision Amazon EC2 capacity in advance. For example, you can set a condition to add new Amazon EC2 instances in increments to the Auto Scaling group when the average utilization of your Amazon EC2 fleet is high; and similarly, you can set a condition to remove instances in the same increments when CPU utilization is low. If you have predictable load changes, you can set a schedule through Auto Scaling to plan your scaling activities. You can use Amazon CloudWatch to send alarms to trigger scaling activities and Elastic Load Balancing to help distribute traffic to your instances within Auto Scaling groups. Auto Scaling enables you to run your Amazon EC2 fleet at optimal utilization.

Reference: http://aws.amazon.com/autoscaling/