A user is planning to use EBS for his DB requirement. The user already has an EC2 instance
running in the VPC private subnet.
How can the user attach the EBS volume to a running instance?

A.
The user can create EBS in the same zone as the subnet of instance and attach that EBS to
instance.

B.
It is not possible to attach an EBS to an instance running in VPC until the instance is stopped.

C.
The user can specify the same subnet while creating EBS and then attach it to a running
instance.

D.
The user must create EBS within the same VPC and then attach it to a running instance.

Explanation:
A Virtual Private Cloud (VPC) is a virtual network dedicated to the user’s AWS account. The user
can create subnets as per the requirement within a VPC. The VPC is always specific to a region.
The user can create a VPC which can span multiple Availability Zones by adding one or more
subnets in each Availability Zone.

The instance launched will always be in the same availability zone of the respective subnet.
When creating an EBS the user cannot specify the subnet or VPC. However, the user must
create the EBS in the same zone as the instance so that it can attach the EBS volume to the
running instance.
http://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/VPC_Subnets.html#VPCSubnet