You have developed an application that processes a massive amount of process logs generated by web site and mobile app. This application requires the ability to analyze petabytes of unstructured data using Amazon Elastic MapReduce. The resultant data is stored on Amazon S3. You have deployed the c4.8xlarge Instance type, whose CPUs are mostly idle during the data processing. Which of the below options would be the most cost-efficient way to reduce the log processing job`s runtime?

Answer options:

A.Create log files with smaller sizes and store them on Amazon S3. Apply the life cycle policy to the S3 bucket such that the files would be first moved to RRS and then to Amazon Glacier vaults.
B.Add additional c4.8xlarge instances by introducing a task instance group. The network performance of 10 Gigabit per EC2 instance would increase the processing speed; thus reducing the load on the EMR cluster.
C.Use smaller instances that have higher aggregate I/O performance.
D.Create fewer, larger log files. Compress and store them on the Amazon S3 bucket. Apply the life cycle policy to the S3 bucket such that the files would be first moved to RRS and then to Amazon Glacier vaults.

Answer – C
Option A is incorrect even though storing the files on an S3 storage class such as RRS would reduce the cost. The problem in the scenario is that the provision of a large instance is wasted due to it being idle most of the time. 
Option B is incorrect as adding more of the c4.8xlarge instance type in the task instance group would create more idle resources, which is - in fact - more costly.
Option C is CORRECT because, since the CPU’s are mostly idle, it means that you have provisioned a larger instance that is under-utilized. A better cost-efficient solution would be to use smaller instances. For batch processing jobs such as the one mentioned in this scenario, you can use multiple t2 instances - which support the concept of CPU bursts - are ideal for situations where there are bursts of CPU during certain periods of time only.
Option D is incorrect even though storing the files on an S3 storage class such as RRS would reduce the cost. The problem in the scenario is that the provision of a large instance is wasted due to it being idle most of the time. 
For more information on resizing of the EC2 instances, please visit the URL given below-
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-instance-resize.html