Answer: Cloud Spanner

The most suitable GCP database for migrating a 20 TB on-premises operational system transaction data is **Cloud Spanner**. Here’s why: - **Cloud Spanner**: It's a globally distributed, scalable, and strongly consistent relational database service designed for mission-critical transactional workloads that require high availability and global scale. It can handle very large datasets, making it ideal for a 20 TB operational system. - **Cloud Bigtable**: If the data can be modeled in a NoSQL wide column format and requires extremely high throughput, Bigtable is an excellent solution. However, for a traditional operational system being moved from on-premises, **Cloud Spanner** is likely the best choice.

Author: LeetQuiz Editorial Team