Answer - C.
The AWS Documentation mentions the following.
DynamoDB Accelerator(DAX) is a DynamoDB-compatible caching service that enables you to benefit from fast in-memory performance for demanding applications.
DAX addresses three core scenarios.
As an in-memory cache, DAX reduces the response times of eventually-consistent read workloads by order of magnitude, from single-digit milliseconds to microseconds.
DAX reduces operational and application complexity by providing a managed service that is API-compatible with Amazon DynamoDB.
Thus it requires only minimal functional changes to use with an existing application.
Option A is invalid because ElastiCache would not guarantee for certain such a great reduction in response times.
The question also asks for a "preferred" service to be used along with DynamoDB, for which DAX would be a better answer than Elasticache.
Option B is invalid because it is used when you want to make replicas of the tables in different regions.
Option D is invalid because this option is used only when you have throttling errors for the table.