The correct choice is A (Cloud Datastore) because it offers a scalable, fully managed NoSQL document database ideal for web and mobile applications. It supports semistructured application data, user profiles, hierarchical data, product catalogs, and durable key-value data, making it perfect for stateful data storage with the ability to scale down to zero.

• B (Cloud Bigtable) is incorrect as it's designed for low-latency single-point lookups and precalculated analytics, not state management.
• C (Cloud Dataproc) is incorrect because it's a service for running Apache Spark and Apache Hadoop clusters, not a storage solution.
• D (Cloud SQL) is incorrect as it requires capacity provisioning and is a managed MySQL and PostgreSQL database service, not suited for non-relational data or scaling down to zero.