You need a globally distributed relational database that scales with unknown user growth and requires minimal configuration changes. Which storage solution should you use?

Cloud Spanner is the correct choice because it's a globally distributed, strongly consistent, relational database service designed for high availability and unlimited horizontal scaling. This makes it ideal for applications with unknown user growth and a need for minimal configuration changes as it scales automatically. Cloud SQL is a regional relational database and doesn't offer global distribution or the same level of automatic scaling. Cloud Firestore and Cloud Datastore are NoSQL document databases, not relational, and while they offer scalability, they don't provide the strong consistency and relational model required.