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

Answer options:

A.Change the thresholds set on the Auto Scaling group health check.
B.Change the type of the health check to "Elastic Load Balancing" in your Auto Scaling group.
C.Increase the value for the Health check interval set on the Elastic Load Balancer.
D.Change the health check on the Elastic Load Balancer to use TCP rather than HTTP checks.

Answer – B
To discover the availability of your EC2 instances, an ELB periodically sends pings, attempts connections, or sends requests to test the EC2 instances. These tests are called health checks. The status of the instances that are healthy at the time of the health check is InService. The status of any instances that are unhealthy at the time of the health check is OutOfService.
When you allow the Auto Scaling group (ASG) to receive the traffic from the ELB, it gets notified when the instance becomes unhealthy, and then it terminates it. See the images in the "More information..." section for more details.
Option A is incorrect because changing the threshold will not enable ASG to know about the unhealthy instances.
Option B is CORRECT because when you associate the ELB with ASG, you allow the ASG to receive the traffic from that ELB. When the health check type is ELB, the ASG will get aware of the unhealthy instances and terminate them.
Option C is incorrect because increasing the interval will still not communicate the information about the unhealthy instances to the ASG.Option D is incorrect because this setting will not communicate the information about the unhealthy instances to the ASG either.
More information on ELB with Auto Scaling Group:
For more information on ELB, please visit the below URL-
https://docs.aws.amazon.com/autoscaling/ec2/userguide/autoscaling-load-balancer.html