The correct option is Operational transactional database.

An operational transactional database is designed for Online Transaction Processing and it is optimized for many small, fast writes and immediate consistency. It provides ACID transactions and row level operations which make it the appropriate place to insert an order event as soon as a buyer completes a subscription.

Managed services on Google Cloud such as Cloud SQL or Cloud Spanner are examples of operational transactional databases you would use to capture order rows and other application events in real time.

Cloud Memorystore is an in memory key value store and cache. It is useful for low latency lookups and session data but it is not intended to be the durable primary store for transactional order records.

BigQuery is an analytical, columnar service built for large scale queries and analytics workloads. It is optimized for scanning and aggregating large datasets rather than for fast, small transactional inserts and immediate consistency.

Data warehouse refers to systems built for reporting and analytics and they are typically used for batch or streamed analytics rather than as the primary transactional store for individual order events.

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