A user has created a blank EBS volume in the US-East-1 region. The user is unable to attach the volume to a running
instance in the same region. What could be the possible reason for this?

A.
The instance must be in a running state. It is required to stop the instance to attach volume

B.
The AZ for the instance and volume are different

C.
The instance is from an instance store backed AMI

D.
The instance has enabled the volume attach protection

Explanation:
An EBS volume provides persistent data storage. The user can attach a volume to any instance provided they are both
in the same AZ. Even if they are in the same region but in a different AZ, it will not be able to attach the volume to that
instance.

http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/AmazonEBS.html