An internal transactional order app requires strong consistency, fast queries, and ACID multi‑table transactions and is currently PostgreSQL with minimal planned code changes. Which Cloud database is most appropriate?

Cloud SQL is the most appropriate choice because it is a fully managed relational database service that supports PostgreSQL, which aligns with the application's current database and minimizes code changes. It provides strong consistency, fast queries, and ACID (Atomicity, Consistency, Isolation, Durability) multi-table transactions, all requirements for the transactional order app. BigQuery is a data warehouse for analytics, not transactional workloads. Cloud Spanner offers global scale and strong consistency but is typically overkill and more complex for an internal app that can be served by a regional database, and it's not PostgreSQL compatible. Cloud Datastore (now Firestore in Datastore mode) is a NoSQL document database, which does not inherently support multi-table ACID transactions in the same way a relational database does, and would require significant code changes.