Your company is getting ready to make a major public announcement of a social media site on AWS. The website runs on EC2 instances deployed across multiple Availability Zones with a Multi-AZ RDS MySQL Extra Large DB Instance. The site performs a high number of small reads and writes per second and relies on an eventual consistency model.

Answer options:

A.Deploy ElasticCache in-memory cache running in each availability zone.
B.Implement sharding to distribute the load to multiple RDS MySQL instances.
C.Increase the RDS MySQL Instance size and implement provisioned IOPS.
D.Add an RDS MySQL read replica in each availability zone.

Answer – A and D
One of AWS pillars of a well-architected framework is cost optimization which incorporates "Right Sizing". AWS defines right Sizing using the lowest cost resource that still meets the technical specifications of the specific workload. Options B and C do not meet those AWS standards.
Option A is CORRECT because ElastiCache is an in-memory caching solution that reduces the load on the database and improves the read performance.
Option B is INCORRECT because splitting the RDS into multiple databases increases both the read and write. Therefore wasting money on increased write capacity is not required. Also, Sharding increases the maintenance overhead as now multiple databases must be maintained and the application must be refactored to incorporate the additional connection strings. 
Option C is INCORRECT because this increases both read and write. Therefore wasting money on increased write capacity which is not required.
Option D is CORRECT because Read Replicas are used to elastically scale out beyond the capacity constraints of a single DB instance for read-heavy database workloads. Hence, improving the read performance.
See more information on Read Replicas and ElastiCache below.
Read Replicas
Amazon RDS Read Replicas provide enhanced performance and durability for database (DB) instances. This replication feature makes it easy to elastically scale out beyond a single DB Instance`s capacity constraints for read-heavy database workloads. You can create one or more replicas of a given source DB Instance and serve high-volume application read traffic from multiple copies of your data, thereby increasing aggregate read throughput.
For more information on Read Replica’s, please visit the below link:
https://aws.amazon.com/rds/details/read-replicas/
ElastiCache
Amazon ElastiCache is a web service that makes it easy to deploy, operate, and scale an in-memory data store or cache in the cloud. The service improves web applications` performance by allowing you to retrieve information from fast, managed, in-memory data stores, instead of relying entirely on slower disk-based databases. 
For more information on Amazon ElastiCache, please visit the below link-
https://aws.amazon.com/elasticache/