Which is the most likely reason for this issue?

Doug has created a VPC with CIDR 10.201.0.0/16 in his AWS account. In this VPC he has created a public subnet with CIDR block 10.201.31.0/24. While launching a new EC2 from the console, he is not able to assign the private IP address 10.201.31.6 to this instance.

Which is the most likely reason for this issue?
A . Private IP address 10.201.31.6 is blocked via ACLs in Amazon infrastructure as a part of platform security.
B . Private address IP 10.201.31.6 is currently assigned to another interface.
C . Private IP address 10.201.31.6 is not part of the associated subnet’s IP address range.
D . Private IP address 10.201.31.6 is reserved by Amazon for IP networking purposes.

Answer: B

Explanation:

In Amazon VPC, you can assign any Private IP address to your instance as long as it is:

Part of the associated subnet’s IP address range

Not reserved by Amazon for IP networking purposes

Not currently assigned to another interface

Reference: http://aws.amazon.com/vpc/faqs/