A user has attached an EBS volume to a running Linux instance as a “/dev/sdf” device. The user is unable to see the
attached device when he runs the command “df -h”. What is the possible reason for this?

A.
The volume is not in the same AZ of the instance

B.
The volume is not formatted

C.
The volume is not attached as a root device

D.
The volume is not mounted

Explanation:
When a user creates an EBS volume and attaches it as a device, it is required to mount the device. If the device/volume
is not mounted it will not be available in the listing.
http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/AmazonEBS.html