You are migrating an existing application to AWS cloud that would be communicating with EC2 instances in the VPC. You need to make this application highly available. The application currently relies on hard-coded host names for communication between the various tiers. You have migrated the application and configured multi-tier using the internal elastic load balancer for serving traffic. The load balancer host name is "demo-app.us-east-1.elb.amazonaws.com". The current hard coded host name in your application for internal communication between your multi-tier application is "demolayer.example.com". How can you architect a solution for high availability?

Answer options:

A.Create an environment variable passed to the EC2 instances using "user-data" with the ELB host name "demo-app.us-east-1.elb.amazonaws.com" 
B.Create a PRIVATE resource record set using Route53 with a host name of "demolayer.example.com" and an ALIAS record to "demo-app.us-east-1.elb.amazonaws.com"
C.Create a PUBLIC resource record set using Route53 with a host name of "demolayer.example.com" and an ALIAS record to "demo-app.us-east-1.elb.amazonaws.com"
D.Add a CNAME record to the existing on-premise DNS server with a value of "demo-app.us-east-1.elb.amazonaws.com". Create a PUBLIC resource record set using Route53 with a host name of "applayer.example.com" and an ALIAS record to "demo-app.us-east-1.elb.amazonaws.com"

Answer – B
Since demolayer.example.com is an internal DNS record, the best way is Route 53 to create an internal resource record. One can then point the resource record to the create ELB.While ordinary Amazon Route 53 resource record sets are standard DNS resource record sets, alias resource record sets provide an Amazon Route 53–specific extension to DNS functionality. Instead of an IP address or a domain name, an alias resource record set contains a pointer to a CloudFront distribution, an Elastic Beanstalk environment, an ELB Classic or Application Load Balancer, an Amazon S3 bucket that is configured as a static website, or another Amazon Route 53 resource record set in the same hosted zone. 
Option A is incorrect because it does not mention how to map between the existing hard-coded hostname and the ELB hostname.
Option B is CORRECT because it creates an internal ALIAS recordset where it defines the mapping between the hard-coded hostname and the ELB hostname that is to be used.
Options C and D are incorrect because they should create a private recordset, not public since the mapping between the hard-coded hostname and ELB hostname should be done internally.
For more information on alias and non-alias records, please refer to the below link-
http://docs.aws.amazon.com/Route53/latest/DeveloperGuide/resource-record-sets-choosing-alias-non-alias.html