What is the reason for this?

You are setting up a very complex financial services grid and so far it has 5 Elastic IP (EIP) addresses. You go to assign another EIP address, but all accounts are limited to 5 Elastic IP addresses per region by default, so you aren’t able to.

What is the reason for this?
A . For security reasons.
B . Hardware restrictions.
C . Public (IPV4) internet addresses are a scarce resource.
D . There are only 5 network interfaces per instance.

Answer: C

Explanation:

Public (IPV4) internet addresses are a scarce resource. There is only a limited amount of public IP space available, and Amazon EC2 is committed to helping use that space efficiently.

By default, all accounts are limited to 5 Elastic IP addresses per region. If you need more than 5 Elastic IP addresses, AWS asks that you apply for your limit to be raised. They will ask you to think through your use case and help them understand your need for additional addresses.

Reference: http://aws.amazon.com/ec2/faqs/#How_many_instances_can_I_run_in_Amazon_EC2