Your application has very high traffic. So you have enabled autoscaling in multi-availability zone to suffice your application`s needs. But you observe that one of the availability zones is not receiving any traffic. What can be wrong here?

Answer options:

A.Autoscaling only works for single availability zone.
B.Autoscaling can be enabled for multi AZ only in north Virginia region.
C.Availability zone is not added to Elastic load balancer.
D.Instances need to manually added to availability zone.

Answer – C
In order to make sure that all the EC2 instances behind a cross-zone ELB receive the requests, make sure that all the applicable availability zones (AZs) are added to that ELB.Option A is incorrect because autoscaling can work with multiple AZs.
Option B is incorrect because autoscaling can be enabled for multi-AZ in any single region, not just N. Virginia. (see the image below)
Option C is CORRECT because most likely, the reason is that the AZ – whose EC2 instances are not receiving requests – is not added to the ELB. 
Option D is incorrect because instances need not be added manually to AZ (they should already be there!).
More information on adding AZs to ELB
When you add an Availability Zone to your load balancer, Elastic Load Balancing creates a load balancer node in the Availability Zone. Load balancer nodes accept traffic from clients and forward requests to the healthy registered instances in one or more Availability Zones.
For more information on adding AZ’s to ELB, please refer to the below URL-
http://docs.aws.amazon.com/elasticloadbalancing/latest/classic/enable-disable-az.html