You need to set up EC2 instances inside a VPC. The requirement is also to create a standby interface if any of the EC2 instances do not respond to traffic. How can you achieve this?

Answer options:

A.Assign a secondary IP to the ENI attached to the EC2 Instance.
B.Attach a public and private IP to the instance.
C.Attach an elastic IP to the Instance.
D.Attach a secondary ENI to the Instance.

Answer - D
AWS Docs provides the following details:
Scenarios for Network Interfaces
Attaching multiple network interfaces to an instance is useful when you want to:
Create a management network.
Use network and security appliances in your VPC.Create dual-homed instances with workloads/roles on distinct subnets.
Create a low-budget, high-availability solution.
Creating a Management Network
You can create a management network using network interfaces. In this scenario, the primary network interface (eth0) on the instance handles public traffic, and the secondary network interface (eth1) handles backend management traffic. It is connected to a separate subnet in your VPC that has more restrictive access controls. The public interface, which may or may not be behind a load balancer, has an associated security group that allows access to the server from the internet (for example, allow TCP port 80 and 443 from 
0.0.0.0/0
, or from the load balancer) while the private facing interface has an associated security group allowing SSH access only from an allowed range of IP addresses either within the VPC or from the internet, a private subnet within the VPC or a virtual private gateway.
To ensure failover capabilities, consider using a secondary private IPv4 for incoming traffic on a network interface. In the event of an instance failure, you can move the interface and/or secondary private IPv4 address to a standby instance.
Options A, B and C are incorrect since none of these options will help accomplish the requirement.
For more information on using the Elastic Network Interface, please refer to the below URL
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-eni.html