AWS Certified Solutions Architect – Associate (SAA-C02) — Question 21

A company is running an ecommerce application on Amazon EC2. The application consists of a stateless web tier that requires a minimum of 10 instances, and a peak of 250 instances to support the application's usage. The application requires 50 instances 80% of the time.
Which solution should be used to minimize costs?

Answer options

• A. Purchase Reserved Instances to cover 250 instances.
• B. Purchase Reserved Instances to cover 80 instances. Use Spot Instances to cover the remaining instances.
• C. Purchase On-Demand Instances to cover 40 instances. Use Spot Instances to cover the remaining instances.
• D. Purchase Reserved Instances to cover 50 instances. Use On-Demand and Spot Instances to cover the remaining instances.

Correct answer: D

Explanation

Option D is the best choice because it allows for a balance of cost-effective Reserved Instances for the consistent 50 instances required most of the time, while using On-Demand and Spot Instances to handle the fluctuations in demand. Option A is too costly as it reserves 250 instances for peak usage which is not always needed. Options B and C do not fully account for the sustained demand of 50 instances, making them less efficient.