Your application has an Auto Scaling group of three EC2 instances behind an Elastic Load Balancer. Your Auto Scaling group was updated with a new launch configuration that refers to an updated AMI. During the deployment, customers complained that they were receiving several errors even though all instances passed the ELB health checks. How can you prevent this from happening again?

Answer options:

A.Create a new ELB and attach the Autoscaling Group to the ELB
B.Create a new launch configuration with the updated AMI and associate it with the AutoScaling group. Increase the size of the group to six and when instances become healthy revert to three.
C.Manually terminate the instances with the older launch configuration.
D.Update the launch configuration instead of updating the Autoscaling Group

Answer - B
An Auto Scaling group is associated with one launch configuration at a time, and you can`t modify a launch configuration after you`ve created it. To change the launch configuration for an Auto Scaling group, you can use an existing launch configuration as the basis for a new launch configuration and then update the Auto Scaling group to use the new launch configuration.
After you change the launch configuration for an Auto Scaling group, any new instances are launched using the new configuration options, but existing instances are not affected.
Then to ensure the new instances are launches , change the size of the Autoscaling Group to 6 and once the new instances are launched, change it back to 3.
For more information on instances scale-in process and Auto Scaling Group`s termination policies please view the following link:
https://docs.aws.amazon.com/autoscaling/ec2/userguide/as-instance-termination.html#default-termination-policy
For more information on changing the launch configuration please see the below link:
http://docs.aws.amazon.com/autoscaling/latest/userguide/change-launch-config.html