you need to do to ensure trial instances marked unhealthy by the ELB will be terminated and replaced?

Question says “instances launched via the Auto Scaling group are being marked unhealthy due to an ELB health check” so we need to tell auto scaling to check ELB health checks for determining health of the instance.

An Auto Scaling instance is either healthy or unhealthy. Auto Scaling determines the health status of an instance using one or more of the following:

1. Status checks provided by Amazon EC2. For more information, see Status Checks for Your Instances in the Amazon EC2 User Guide for Linux Instances.
2. Health checks provided by Elastic Load Balancing. For more information, see Health Checks for Your Target Groups in the Application Load Balancer Guide or Configure Health Checks for Your Classic Load Balancer in the Classic Load Balancer Guide.
3. Custom health checks. For more information, see Instance Health Status and Custom Health Checks.

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 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.
Amazon AWS-SysOps : Practice Test
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