You are designing a system that needs at minimum, 8 m4.large instances operating to service traffic. While designing a system for high availability in the us-east-1 region having 6 Availability Zones, your company needs to handle the death of a full availability zone. How should you distribute the EC2 servers in a cost-effective way?

Answer options:

A.3 servers in each of AZs a through d, inclusive.
B.16 servers in AZa.
C.2 servers in each of AZs a through e, inclusive.
D.4 servers in each of AZs a through c, inclusive.

Correct Answer – C
The best way is to distribute the instances across multiple AZs to get the best performance and avoid a disaster scenario.
With this solution, you will always have a minimum of more than 8 servers, even if one AZ went down.
Even though options A and D are also valid, they need more EC2 instances than Option
C.For more information on High Availability and Fault tolerance, please refer to the link below.
https://media.amazonwebservices.com/architecturecenter/AWS_ac_ra_ftha_04.pdf
Note: 
In option A, we need to distribute 3 servers in each AZ.
So,A=3, B=3, C=3, D=3. Total servers used=12.
In option B, we will not get high availability. There will be no servers available if AZa is down.
In option C, we need to distribute 2 servers in each AZ.
so, A=2, B=2, C=2, D=2, E=2. Total servers used=10.
In option D, we need to distribute 4 servers in each AZ.
So, A=4, B=4, C=4. Total servers used=12.
In the question, a cost-effective method is required. In option C, we are using fewer servers, i.e., 10 servers distributed in more AZ`s.