Amazon EFS is a regional, elastic file system that “scales to petabytes” and can be accessed from “thousands of compute instances” concurrently. You can mount EFS across VPCs and across AWS accounts by providing network connectivity (e.g., VPC peering) to the EFS mount targets and allowing NFS (TCP 2049) in the mount target security group, optionally using EFS access points and a file system policy for cross-account access control. VPC peering has no hourly charge and minimal operational overhead, and EFS automatically scales as files are added—meeting the scalability and cost goals.

Option A duplicates data, incurs DataSync and extra storage costs, and adds sync lag.

Option C adds an extra Lambda hop and complexity without exposing a shared filesystem to the primary function.

Option D is impractical: Lambda layers are immutable artifacts with tight size limits (up to 50 MB compressed/250 MB uncompressed) and are not suited for dynamic, growing file sets.