To meet the requirements of 99.99% availability, global transactional consistency, and the need for a fully managed relational database that supports read and write instances in multiple regions, Cloud Spanner is the most suitable option. Cloud Spanner is a globally distributed database service that provides high availability, strong consistency, and scalability. Bigtable is a NoSQL database that does not support relational data models or global transactional consistency. Firestore is also a NoSQL database and lacks the relational capabilities and global consistency required. Cloud SQL for MySQL is a relational database but does not natively support global distribution and the high availability across multiple regions as effectively as Cloud Spanner.