Choose a fully managed, auto-scaling, transactionally consistent, SQL-queryable database that can scale to 6 TB.

Cloud SQL is the correct choice because it is a fully managed relational database service that supports SQL, offers transactional consistency, and can scale up to 64 TB (well beyond the 6 TB requirement). While it doesn't auto-scale compute in the same way as some other services, it handles storage auto-scaling. Cloud Bigtable is a NoSQL wide-column store, not SQL-queryable. Cloud Spanner is a globally distributed, strongly consistent relational database, but it's designed for petabyte-scale and global consistency, making it overkill and more expensive for a 6 TB regional requirement, and it's not simply "SQL-queryable" in the traditional sense without specific client libraries. Cloud Datastore (now Firestore in Datastore mode) is a NoSQL document database, not SQL-queryable.