A Lambda function runs in a VPC and is triggered by S3 object uploads. The function writes result files and appends entries to a shared log file that must be accessible to other Lambda functions, AWS services, and on-premises systems. Which storage solution meets these shared-file and append requirements?

Amazon EFS is the correct solution because it provides a scalable, shared file system that can be mounted by multiple Lambda functions and accessed by other AWS services and on-premises systems. This directly addresses the requirement for a shared log file and the ability to append entries. EBS Multi-Attach volumes are designed for EC2 instances, not Lambda functions, and the "download, append, and upload" mechanism is inefficient and complex for this use case. The /tmp directory in Lambda is ephemeral storage, meaning data is lost between invocations, making it unsuitable for persistent shared logs. The /opt directory is read-only and used for deployment packages, not for runtime data storage.