What are the steps that get carried out by OpsWork when you attach a load balancer to a layer in OpsWork?

Answer options:

A.Terminates the EC2 Instances.
B.De-registers any currently registered instances.
C.Automatically registers the layer`s instances when they come online and de-registers instances when they go offline, including load-based and time-based instances.
D.Automatically starts routing requests to registered instances in their Availability Zones.

Answer– B, C, and D
For the exam, remember that, after you attach a load balancer to a layer, AWS OpsWorks Stacks does the following.
Deregisters any currently registered instances. 
Automatically registers the layer`s instances when they come online and deregisters instances when they leave the online state, including load-based and time-based instances.
Automatically activates and deactivates the instances` Availability Zones.
Hence, options B, C, and D are CORRECT.
For more information on working with Opswork layer ELB’s, please refer to the below link
http://docs.aws.amazon.com/opsworks/latest/userguide/layers-elb.html
Please refer to page 218 on the below link
https://docs.aws.amazon.com/opsworks/latest/userguide/opsworks-ug.pdf