You are planning to create a fault-tolerant EC2 Instance by creating a secondary network interface and a backup EC2 Instance. Which of the following is a requirement to ensure the switch over can be done successfully? (Select TWO)

Answer options:

A.The network interface must reside in the same Availability Zone.
B.The network interface must reside in a different Availability Zone.
C.The instance must reside in the same Availability Zone.
D.The instance must reside in a different Availability Zone.

Answer – B and D
This is given in the AWS documentation.
You can attach a network interface in one subnet to an instance in another subnet in the same VPC. 
Options A and C are incorrect because we require fault-tolerant infrastructure that can only be achieved when the network interface and instance must be in the different Availability Zone.
For more information on Elastic Network Interfaces, please refer to the below URL
http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-eni.html
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/MultipleIP.html