A user has launched an EBS backed Linux instance. How can a user detach the root device and attach it to another
instance as a secondary volume?

A.
Unmount the root volume first and then detach it

B.
It is not possible to mount the root volume to some other instance

C.
Stop the first instance and then attach instance’s root volume as a new volume to the other instance

D.
It is not possible to mount the root device as a secondary volume on the other instance

Explanation:
If an Amazon EBS volume is the root device of an instance, it cannot be detached unless the instance is in the stopped
state.
http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ebs-detaching-volume.html