B is correct because Cloud Storage is designed for object storage and can accommodate files of any type, making it ideal for unstructured data. The gsutil tool facilitates interaction with Cloud Storage, offering a scalable and durable solution for storing and accessing such files. This method ensures the data is accessible on Google Cloud for subsequent ETL transformations with Cloud Dataflow.

A is incorrect as BigQuery is tailored for structured data and supports only a limited range of file formats, not suited for unstructured files.

C is incorrect because Cloud SQL is a managed relational database service focused on structured data, not unstructured files.

D is incorrect since Cloud Spanner, a globally distributed relational database, is also structured and not designed for unstructured data handling.

For more information, visit: Google Cloud Storage