A web application is running on AWS. The site runs stateless web servers on three Amazon EC2 instances behind an Application Load Balancer. The company is receiving complaints from some users about the application performance. Amazon CloudWatch shows that one of the EC2 instances runs at approximately 90% CPU utilization, while the other two are running at approximately 20% CPU utilization.
How can a SysOps Administrator prevent this issue from recurring?

Answer options:

A.Manually provision a fourth EC2 instance and add it to the Application Load Balancer.
B.Review the server`s performance that supports the web application, then choose a more appropriate instance size and apply the change.
C.Disable sticky sessions on the Application Load Balancer and check CloudWatch to confirm that the issue has been resolved.
D.Review the number of sessions from the Application Load Balancer to each web server. Restart any web server that has significantly more sessions to even out the load.

Correct Answer: C
Here the issue is clearly to do with sticky sessions. Because of this, all sessions are being directed to one server. By disabling the sticky sessions the traffic can be load balanced in all three instances.
The AWS Documentation mentions the following.
By default, an Application Load Balancer routes each request independently to a registered target based on the chosen load-balancing algorithm. However, you can use the sticky session feature (also known as session affinity) to enable the load balancer to bind a user`s session to a specific target. This ensures that all requests from the user during the session are sent to the same target. 
Option A is incorrect because adding the fourth instance will not restrict the traffic to target one instance with 90% utilization.
Option B is incorrect because increasing the instance will allow the other two instances to be underutilized. The load balancer should balance the load of all three instances.
Option D is incorrect because there is no need to check sessions in all the instances. It is better to check sessions for the instance having maximum utilization. Restarting the webserver is not the required solution.
For more information on sticky sessions, please visit the below URL-
https://docs.aws.amazon.com/elasticloadbalancing/latest/application/sticky-sessions.html