You currently have the following architecture in AWS:
a. A couple of EC2 Instances located in us-west-2a
b. The EC2 Instances are launched via an Auto Scaling group.
c.The EC2 Instances sit behind a Classic ELB. 
Which additional step would ensure that the above architecture conforms to a well-architected framework?
 

Answer options:

A.Convert the Classic ELB to an Application ELB.B.Add an additional Auto Scaling Group.
C.Add additional EC2 Instances to us-west-2a.
D.Add or spread existing instances across multiple Availability Zones.

Correct Answer - D
AWS Documentation provides the following information to support this concept:
Balancing resources across Availability Zones is a best practice for well-architected applications, as this greatly increases aggregate system availability. Auto Scaling automatically balances EC2 instances across zones when you configure multiple zones in your Auto Scaling group settings. Auto Scaling always launches new instances such that they are balanced between zones as evenly as possible across the entire fleet. 
For more information on managing resources with Auto Scaling, please refer to the URL below.
https://aws.amazon.com/blogs/compute/fleet-management-made-easy-with-auto-scaling/