You need your CI to build AMIs with code pre-installed on the images on every new code push. You need to do this as cheaply as possible. How do you do this?

Answer options:

A.Create a spot instance request to include the maximum price that you are willing to pay per hour per instance just above the spot price as soon as new commits come in, perform all instance configuration and setup, then create an AMI based on the spot instance.
B.Have the CI launch a new on-demand EC2 instance when new commits come in, perform all instance configuration and setup, then create an AMI based on the on-demand instance.
C.Purchase a Light Utilization Reserved Instance to save money on the continuous integration machine.Use these credits whenever you create AMIs on instances.
D.When the CI instance receives commits, attach a new EBS volume to the CI machine. Perform all setup on this EBS volume. So you don’t need a new EC2 instance to create the AMI.

Answer – A
Amazon EC2 Spot instances allow you to make use of spare Amazon EC2 computing capacity. Since Spot instances are often available at a discount compared to On-Demand pricing, you can significantly reduce the cost of running your applications, grow your application’s compute capacity and throughput for the same budget, and enable new types of cloud computing applications.
For more information on Spot Instances, please visit the below URL:
https://aws.amazon.com/ec2/spot/
Note: Since we are quoting a price higher than the spot price, the most likelihood of terminating the instance due to the price fluctuation seems to be quite unlikely to occur. Since the question`s focus is on the cost factor, this is the most suitable option.
Light utilization RIs are not currently available for purchase. So it is not a valid option. Although the availability cannot be promised with Spot instances, and there can also be interruptions in between, it is still being used by many businesses because of its cost-efficiency. 
To make use of Spot instances, there are various practices that we can follow to ensure less interruptions.
We can use pools with lower prices and more stable prices (both current and historical) to find bargains and lower interruption rates. Check the price history of the instance types, make the bid price higher than the current price, usage of multiple capacity pools etc are various approaches that we can practice to reduce the interruptions on Spot instances.