You currently have the following setup in AWS

Answer options:

A.Create a second ELB, and a new Auto Scaling Group assigned a new Launch Configuration with the new AMI. Use the Route53 Weighted Routing policy to adjust the proportion of traffic hitting the two ELBs.
B.Create new AMIs with the new app. Then use the new EC2 instances in half proportion to the older instances.
C.Redeploy with AWS Elastic Beanstalk and Elastic Beanstalk versions. Use Route 53 Weighted Round Robin Records to adjust the proportion of traffic hitting the two ELBs.
D.Create a full second stack of instances, cut the DNS over to the new stack of instances, and change the DNS back if a rollback is needed.

Answer - A
The Weighted Routing policy of Route53 can be used to direct a proportion of traffic to your application. The best option is to create a second ELB, attach the new Autoscaling Group, and then use Route53 to divert the traffic.
Option B is wrong because it does not mention how to divide the instances. If it is the same Auto Scaling group, it will be difficult to revert the configurations back.
Option C is wrong because Elastic beanstalk is good for development environments. Also, there is no mention of having 2 environments where environment URLs can be swapped.
Option D is wrong because you cannot control the weight for either the Blue or Green stack with this method.
For more information on Route53 routing policies, please refer to the below link:
http://docs.aws.amazon.com/Route53/latest/DeveloperGuide/routing-policy.html