You currently have an application deployed via Elastic Beanstalk. You are now deploying a new application. In Elastic Beanstalk, the current instances are detached, and new instances are deployed and reattached. But the new instances are still not receiving any sort of traffic immediately. Why does this happen?

Answer options:

A.The instances are of the wrong AMI. Hence they are not detected by the ELB.B.It takes time for the ELB to register the instances. Hence there is a small time frame before your instances can start receiving traffic.
C.You need to create a new Elastic Beanstalk application, because you cannot detach and then reattach instances to an ELB within the same Elastic Beanstalk application.
D.The instances need to be reattached before the new application version is deployed.

Answer - B
Before the EC2 Instances can start receiving traffic, they will be checked via the health checks of the ELB. Once the health checks are successful, the EC2 Instances will change the state to InService and then the EC2 Instances can start receiving traffic.
For more information on ELB health checks, please refer to the below link:
http://docs.aws.amazon.com/elasticloadbalancing/latest/classic/elb-healthchecks.html