You have multiple EC2 instances in three availability zones (AZs), with a load balancer configured for your application. Any instance can process the requests and there is no need to route the requests for a particular session to the same EC2 instance. However, you observe that only one of those AZs is receiving all the traffic. Which of the following actions may help distribute the traffic in three AZs? (Select TWO).

Answer options:

A.Disable sticky sessions
B.Reduce the frequency of the health checks
C.Enable cross zone load balancer
D.Amazon recommends to use two availability zone behind ELB

Answer – A, C
Since the traffic is routed to only one availability zone (AZ) and none of the other AZs are receiving any, the ELB must have only one AZ registered in it. First, you have to ensure that the ELB is configured to support multiple AZs via Cross-Zone load balancing. Even after enabling the cross zone load balancing, if the traffic is routed to particular EC2 instances in an AZ, the users` sessions must have tied to those EC2 instances. These symptoms seem to be related to the sticky sessions (session affinity). So, the second thing you must ensure that the sticky sessions need to be either disabled or configured to be expiring after a specific period.
Option A is CORRECT because, as mentioned above, sticky sessions could be a reason for traffic being routed to specific EC2 instances in a specific AZ.
Option B is incorrect because reducing the health check frequency will not balance the traffic between different AZs.
Option C is CORRECT because cross zone load balancing needs to be enabled on the ELB and the other AZs must be registered under this ELB.Option D is incorrect because there is no such recommendation from Amazon about ELB.More information on ELB, Sticky Sessions, and Cross Zone Load Balancing:
https://docs.aws.amazon.com/elasticloadbalancing/latest/classic/enable-disable-crosszone-lb.html
https://docs.aws.amazon.com/elasticloadbalancing/latest/classic/elb-sticky-sessions.html