An EC2 instance has one additional EBS volume attached to it. How can a user attach the same volume to another
running instance in the same AZ?

A.
Terminate the first instance and only then attach to the new instance

B.
Attach the volume as read only to the second instance

C.
Detach the volume first and attach to new instance

D.
No need to detach. Just select the volume and attach it to the new instance, it will take care of mapping internally

Explanation:
If an EBS volume is attached to a running EC2 instance, the user needs to detach the volume from the original instance
and then attach it to a new running instance. The user doesn’t need to stop / terminate the original instance.
http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ebs-detaching-volume.html