You must store relational data for a global user base with unknown growth and minimal operational changes. Which storage solution scales accordingly?

Cloud Spanner is the correct choice because it's a globally distributed, strongly consistent, relational database service that offers unlimited horizontal scaling, making it ideal for unknown growth and global user bases. Its managed nature also minimizes operational overhead. Cloud SQL is a regional relational database and doesn't offer the same global distribution or unlimited scaling. Firestore is a NoSQL document database, not relational, and while it scales well, it doesn't provide strong relational consistency across a global scale like Spanner. Bigtable is a NoSQL wide-column database designed for analytical workloads and high-throughput reads/writes, not for relational data with strong consistency requirements.