Your company stores millions of sensitive transactions across thousands of 100-GB files that must be encrypted in-transit and at-rest. Analysts concurrently depend on subsets of files, which can consume up to 35 TB of space, to generate simulations that can be used to steer business decisions. You are required to design an AWS solution that can cost-effectively accommodate the long-term storage and in-flight subsets of data. Which one would you choose? 

Answer options:

A.Use Amazon Simple Storage Service (S3) with server-side encryption, and run simulations on subsets in ephemeral drives on Amazon EC2.
B.Use Amazon S3 with server-side encryption, and run simulations on subsets in-memory on Amazon EC2.
C.Use HDFS on Amazon EMR, and run simulations on subsets in ephemeral drives on Amazon EC2.
D.Use HDFS on Amazon Elastic MapReduce File System (EMRFS) in conjunction with AWS S3. 
E.Store the full data set in encrypted Amazon Elastic Block Store (EBS) volumes and regularly capture snapshots that can be cloned to EC2 workstations.

Answer - D
The main considerations of this scenario are: (1) the solution must be cost-effective, (2) provide long-term storage, and (3) encrypt in-transit as well as at-rest data.
Option A is incorrect because (a) server-side encryption does not apply to in-transit data, and (b) ephemeral volumes are not encrypted at rest.
Option B is incorrect. Yes, Amazon S3 supports server-side encryption, but 35TB in memory not possible because the largest instances have only 25 TB of capacity. Due to this, we have to mark this option wrong.
Option C is incorrect because the ephemeral drive is not long term storage.
Option D is CORRECT because (a) EMR File system supports both in-transit and at-rest data encryption, and (b) S3 provides the long term storage.
Option E is incorrect because this is not a cost-effective solution.
For more information on EMR, please visit the link below:
https://aws.amazon.com/blogs/aws/new-at-rest-and-in-transit-encryption-for-amazon-emr/
https://d0.awsstatic.com/whitepapers/aws-amazon-emr-best-practices.pdf
https://docs.aws.amazon.com/emr/latest/ManagementGuide/emr-data-encryption-options.html ( refer section "Encryption at Rest for EMRFS Data in Amazon S3" )