Explanation

On-Demand Instances are the most cost-effective choice for this scenario because:

• Short-term usage: The instances are only needed for exactly 1 month with no long-term requirement
• No upfront commitment: On-Demand instances don't require any upfront payment or long-term contract
• Continuous availability: Unlike Spot Instances, On-Demand instances won't be interrupted
• Cost comparison:
  • Reserved Instances require 1-3 year commitments and upfront payments, which are not cost-effective for just 1 month
  • Dedicated Hosts are physical servers dedicated to your use, typically more expensive and require longer commitments
  • Spot Instances are cheaper but can be interrupted with little notice, making them unsuitable for continuous quality assurance tests

For short-term, predictable workloads where continuous availability is required, On-Demand instances provide the best balance of cost and reliability without long-term commitments.