What is a possible reason that the instance is unavailable in the assigned IP console?

A user has created a subnet in VPC and launched an EC2 instance within it. The user has not selected the option to assign the IP address while launching the instance. The user has 3 elastic IPs and is trying to assign one of the Elastic IPs to the VPC instance from the console. The console does not show any instance in the IP assignment screen.

What is a possible reason that the instance is unavailable in the assigned IP console?
A . The IP address may be attached to one of the instances
B . The IP address belongs to a different zone than the subnet zone
C . The user has not created an internet gateway
D . The IP addresses belong to EC2 Classic; so they cannot be assigned to VPC

Answer: D

Explanation:

A Virtual Private Cloud (VPC) is a virtual network dedicated to the user’s AWS account. A user can create a subnet with VPC and launch instances inside that subnet. When the user is launching an instance he needs to select an option which attaches a public IP to the instance. If the user has not selected the option to attach the public IP then it will only have a private IP when launched. If the user wants to connect to an instance from the internet he should create an elastic IP with VPC. If the elastic IP is a part of EC2 Classic it cannot be assigned to a VPC instance.

Reference: http://docs.aws.amazon.com/AmazonVPC/latest/GettingStartedGuide/LaunchInstance.html