Elastic IP

Answer - A.

The AWS documentation mentions the following.

An Elastic IP address is a property of network interfaces.

You can associate an Elastic IP address with an instance by updating the network interface attached to the instance.

For more information on Elastic IP's, please refer to the below link:

The Elastic IP (EIP) is a static, public IPv4 address that can be associated with an instance in the Amazon Web Services (AWS) cloud. It is not tied to a specific network interface, instance or subnet and can be re-associated with any instance in a particular AWS account, as long as it is not already associated with another resource.

Therefore, the correct answer is A. Elastic Network Interface.

An Elastic IP can be associated with a specific Elastic Network Interface (ENI) attached to an instance or to a Network Load Balancer (NLB) in the Virtual Private Cloud (VPC) network. When an Elastic IP is associated with an ENI, the instance can communicate over the internet with a static public IP address even after the instance is stopped or restarted.

A subnet is a range of IP addresses in a VPC, and does not have the ability to hold an Elastic IP. CloudTrail is a service used to log, monitor, and retain account activity related to API calls within the AWS infrastructure, and does not relate to Elastic IPs. Lastly, a MAC address is a unique identifier assigned to network interfaces for communications on the physical network, and is not associated with Elastic IPs in any way.

In summary, Elastic IP is a property of Elastic Network Interface (ENI) and can be associated with an instance or Network Load Balancer (NLB) in the VPC network to provide a static, public IPv4 address for communication over the internet.