You have an application running in us-west-2 that requires 6 EC2 Instances running at all times. With 3 Availability Zones in the region viz. us-west-2a, us-west-2b, and us-west-2c, which of the following deployments provides fault tolerance if ONE Availability Zone in us-west-2 becomes unavailable? (SELECT TWO.)

Answer options:

A.2 EC2 Instances in us-west-2a, 2 EC2 Instances in us-west-2b, and 2 EC2 Instances in us-west-2c
B.3 EC2 Instances in us-west-2a, 3 EC2 Instances in us-west-2b, and no EC2 Instances in us-west-2c
C.4 EC2 Instances in us-west-2a, 2 EC2 Instances in us-west-2b, and 2 EC2 Instances in us-west-2c
D.6 EC2 Instances in us-west-2a, 6 EC2 Instances in us-west-2b, and no EC2 Instances in us-west-2c
E.3 EC2 Instances in us-west-2a, 3 EC2 Instances in us-west-2b, and 3 EC2 Instances in us-west-2c

Answer – D and E
Option D- US West 2a-6 , US West 2b - 6, US West 2c-0
If US West 2a goes down, we will still have 6 instances running in US West 2b. 
If US West 2b goes down, we will still have 6 instances running in US West 2a.
If US West 2c goes down, we will still have 6 instances running in US West 2a, 6 instances running in US West 2b.
Option E- US West 2a-3 , US West 2b - 3, US West 2c-3
If US West 2a goes down, we will still have 3 instances running in US West 2b and 3 instances running in US West 2c. 
If US West 2b goes down, we will still have 3 instances running in US West 2a and 3 instances running in US West 2c. 
If US West 2c goes down, we will still have 3 instances running in US West 2a and 3 instances running in US West 2b. 
Option A is incorrect because, even if one AZ becomes unavailable, we will only have 4 instances available. This does not meet the specified requirements.
Option B is incorrect because, when either us-west-2a or us-west-2b is unavailable, you would only have 3 instances available. This does not meet the specified requirements.
Option C is incorrect because, if us-west-2a becomes unavailable, you would only have 4 instances available. This also does not meet the specified requirements.
For more information on AWS Regions and Availability Zones, please visit the following URL:
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Concepts.RegionsAndAvailabilityZones.html
Note: 
In this scenario, we need to have 6 instances running all the time, even when 1 AZ is down.
Hence options D & E are correct.