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 GroupBy 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 Health Check 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 Load-Balanced 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.