Which AWS service is not primarily used for storage solutions?

Amazon ElastiCache is the correct answer because it is primarily designed for in-memory caching to improve the performance of applications by providing fast access to data. It supports data storage but is not intended as a primary data storage solution like the other options, which focus on persistent data storage.

Amazon EFS (Elastic File System) provides scalable file storage for use with Amazon EC2 instances. It is designed to store and manage large amounts of file data.

Amazon Glacier (now part of Amazon S3 Glacier) is a dedicated storage service for long-term data archiving. It is optimized for data that is infrequently accessed and is known for its low-cost storage capabilities.

Amazon RDS (Relational Database Service) is used for setting up, operating, and scaling relational databases, which inherently involves storage for structured data.

In summary, while all the other services involve storing data, Amazon ElastiCache stands out because it is specifically engineered to enhance application performance through caching rather than providing long-term data storage.