An Amazon EMR cluster with four nodes is running 24/7/365 and expects potentially to add one extra node for several days only during the entire year that can be terminated. Which architecture would have the lowest possible cost for the cluster requirement?

Answer options:

A.Purchase 4 reserved nodes and rely on on-demand instances for the fifth node, if required.
B.Purchase 5 reserved nodes to cover all possible usage during the year.
C.Purchase 4 reserved nodes and launch a spot instance for the extra node if required.
D.Purchase 2 reserved nodes and utilize 3 on-demand nodes only for peak usage times.

Answer - C
Option A is INCORRECT because purchasing On-demand instances would involve more cost than purchasing a spot instance.
Option B is incorrect because reserving the 5th node is unnecessary. 
Option C is CORRECT because (a) the application requires 4 nodes throughout the year and reserved instances would save the cost and (b) since the extra node needs only to run for several days in a year and it can be terminated, purchasing a spot instance can provide a lowest cost option in addition to purchasing RI.
Option D is incorrect because reserving only 2 instances would not be sufficient.
Please find the below link for Reserved Instances.
https://aws.amazon.com/ec2/pricing/reserved-instances/