The correct answer is Data stored persistently on disks, optical media, or cloud object storage.

This phrase refers to information that resides on physical storage or in cloud object stores when it is not being processed or transmitted. Examples include files on disks, database records, backups, and blobs in object storage. Protecting this stored data typically involves encryption at rest, access controls, and secure key management to prevent unauthorized access to persisted content.

Data moving across a network between systems is incorrect because that describes data in transit. Data in transit is protected with transport level controls such as TLS and secure networking rather than the controls used for data at rest.

Data held temporarily in a system's RAM or CPU cache is incorrect because that describes data in use. Data in memory is ephemeral and is addressed with different protections like memory encryption and runtime isolation.

Data queued in a write buffer while awaiting final persistence is incorrect because write buffers are transient and part of ongoing input output operations. Queued data is considered in flight or in use until it is committed to persistent storage and is not classified as data at rest.

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