You are responsible for maintaining a multi-regional storage solution for an application that serves millions of users worldwide. The solution needs to have low-latency read and write operations, global transaction consistency, and scalability. Which GCP service is the most appropriate choice?

*Explanation:* Cloud Spanner with multi-region configurations. Cloud Spanner is the most appropriate choice for this scenario. It offers low-latency reads and writes, global transaction consistency, and scalability across multiple regions, making it well-suited for applications serving users worldwide. While other services like Cloud Bigtable, Cloud Storage, Cloud SQL, and Cloud Firestore have their strengths, they do not provide the same combination of features required for this specific use case.