The correct answer is Cloud Bigtable. This GCP service is engineered to deliver a scalable, fully managed NoSQL database, capable of handling vast amounts of both structured and unstructured data efficiently, with minimal latency and high throughput. It's particularly suited for applications requiring real-time access to large datasets.

• Cloud Datastore is another NoSQL option within GCP, but it's not optimized for the same level of scale or performance as Cloud Bigtable.
• Cloud Spanner stands out as a globally distributed, horizontally scalable relational database, distinguishing it from NoSQL solutions.
• Cloud SQL provides a managed service for relational databases, supporting SQL-based systems like MySQL and PostgreSQL, and thus, does not qualify as a NoSQL database.

Given these distinctions, Cloud Bigtable is the definitive choice for a scalable and fully managed NoSQL database service in GCP.