You have an Auto Scaling group associated with an Elastic Load Balancer (ELB). You have noticed that
instances launched via the Auto Scaling group are being marked unhealthy due to an ELB health check, but
these unhealthy instances are not being terminated.
What do you need to do to ensure trial instances marked unhealthy by the ELB will be terminated and
replaced?

A.
Change the thresholds set on the Auto Scaling group health check

B.
Add an Elastic Load Balancing health check to your Auto Scaling group

C.
Increase the value for the Health check interval set on the Elastic Load Balancer

D.
Change the health check set on the Elastic Load Balancer to use TCP rather than HTTP checks

Explanation:
http://docs.aws.amazon.com/AutoScaling/latest/DeveloperGuide/as-add-elb-healthcheck.html
Add an Elastic Load Balancing Health Check to your Auto Scaling Group
By default, an Auto Scaling group periodically reviews the results of EC2 instance status to determine the
health state of each instance. However, if you have associated your Auto Scaling group with an Elastic Load
Balancing load balancer, you can choose to use the Elastic Load Balancing health check. In this case, Auto
Scaling determines the health status of your instances by checking the results of both the EC2 instance status
check and the Elastic Load Balancing instance health check.
For information about EC2 instance status checks, see Monitor Instances With Status Checks in the Amazon
EC2 User Guide for Linux Instances. For information about Elastic Load Balancing health checks, see HealthCheck in the Elastic Load Balancing Developer Guide.
This topic shows you how to add an Elastic Load Balancing health check to your Auto Scaling group, assuming
that you have created a load balancer and have registered the load balancer with your Auto Scaling group. If
you have not registered the load balancer with your Auto Scaling group, see Set Up a Scaled and LoadBalanced Application.
Auto Scaling marks an instance unhealthy if the calls to the Amazon EC2 action DescribeInstanceStatus return
any state other than running, the system status shows impaired, or the calls to Elastic Load Balancing
action DescribeInstanceHealth returns OutOfService in the instance state field.
If there are multiple load balancers associated with your Auto Scaling group, Auto Scaling checks the health
state of your EC2 instances by making health check calls to each load balancer. For each call, if the Elastic
Load Balancing action returns any state other than InService, the instance is marked as unhealthy. After Auto
Scaling marks an instance as unhealthy, it remains in that state, even if subsequent calls from other load
balancers return an InService state for the same instance.