Correct Answer: C (Cloud Spanner)

• A (Firestore) is incorrect because it is a NoSQL document database, not a relational database. It offers flexibility and scalability but lacks the strong consistency and global replication needed for this scenario.
• B (Cloud SQL) is incorrect as it does not inherently support global availability. It's a traditional relational database service without built-in multi-region replication, potentially leading to higher latency.
• C (Cloud Spanner) is correct because it is a fully managed relational database that provides unlimited scale, strong consistency, and 99.999% availability. It ensures users worldwide see the exact same data with minimal latency by automatically replicating data across multiple regions.
• D (Cloud Bigtable) is incorrect as it is a NoSQL database offering eventual consistency, not suitable for applications requiring strong consistency and global data uniformity.

Cloud Spanner is designed for mission-critical applications requiring transactional consistency at a global scale, making it the optimal choice for this forex trading application.