An organization has an ecommerce application that requires a cost-effective, transactional database. The application will only serve customers in a single region. Which service should they use?

BigQuery is a data warehouse optimized for analytical (OLAP) queries, not ACID-compliant transactional ecommerce workloads.

Cloud Bigtable is a NoSQL wide-column store designed for high-throughput, low-latency reads and writes on large datasets, not relational transactional workloads.

Cloud Spanner provides globally distributed transactions and is far more expensive than Cloud SQL, making it cost-inefficient for a single-region application.

Cloud SQL is a fully managed relational database service supporting MySQL, PostgreSQL, and SQL Server that is optimized for transactional (OLTP) workloads in a single region. It is significantly more cost-effective than Cloud Spanner for single-region deployments, and it natively supports ACID transactions required by ecommerce applications. Its managed nature reduces operational overhead while meeting the stated regional and transactional requirements.