Where is the session data best written so that it can be served reliably across multiple requests?

An application runs on multiple EC2 instances behind an ELB.
Where is the session data best written so that it can be served reliably across multiple requests?
A. Write data to Amazon ElastiCache
B. Write data to Amazon Elastic Block Store.
C. Write data to Amazon EC2 Instance Store.
D. Write data to the root filesystem.