You have a distributed application that periodically processes large volumes of data across
multiple Amazon EC2 Instances. The application is designed to recover gracefully from
Amazon EC2 instance failures. You are required to accomplish this task in the most costeffective way.
Which of the following will meet your requirements?

A.
Spot Instances

B.
Reserved instances

C.
Dedicated instances

D.
On-Demand instances

Explanation:
Using reserved instances is not the most cost-effective way.
https://aws.amazon.com/blogs/aws/new-scheduled-reserved-instances/
“Scheduled Reserved Instance model allows you to reserve instances for predefined blocks of
time on a recurring basis for a one-year term, with prices that are generally 5 to 10% lower
than the equivalent On-Demand rates.”

You can get spot instances with much lower prices:
https://aws.amazon.com/ec2/spot/pricing/
“Spot instances are also available to run for a predefined duration – in hourly increments up to
six hours in length – at a significant discount (30-45%) compared to On-Demand pricing plus
an additional 5% during off-peak times for a total of up to 50% savings.”