The correct answer is Object storage.

Object storage stores data as discrete objects with metadata and unique identifiers which makes it well suited to handle very large single files such as virtual machine images. It scales horizontally and is accessed via HTTP APIs so objects do not need to be attached to a specific host which makes it a cost effective and durable choice for long term retention of VM images.

Unstructured storage is a vague term that describes the nature of data rather than a specific cloud storage model. It does not guarantee the API semantics durability and host independence that Object storage provides so it is not the best answer.

Block volume storage offers low latency block level access and is typically attached to a host for boot disks and databases. That attachment model and its performance orientation make it unsuitable for cost effective long term archival and distribution of large VM image files compared with Object storage.

Structured storage usually refers to databases and systems optimized for records and queries rather than large binary blobs. It is not designed to store and serve very large single files like VM images efficiently so it is not appropriate for this use case.

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