Answer: Use Cloud SQL for PostgreSQL with cross-region replicas.

Given the requirements of the application—global read-only access with nightly updates—the best choice is an open source, SQL-compliant database that can handle global distribution efficiently. Option A, Bigtable, is not SQL-compliant and is more suited for large-scale, low-latency applications. Option B, Memorystore for Redis, is an in-memory data store and not a full-fledged SQL database, making it unsuitable for this scenario. Option C, Cloud SQL for PostgreSQL with cross-region replicas, is SQL-compliant and open source, and cross-region replicas can provide the global read access needed, but it may not be the most cost-effective or scalable solution for a truly global application. Option D, Cloud Spanner with multi-region configuration, is SQL-compliant, globally distributed, and can handle the read-only nature of the application efficiently, making it the best choice despite not being open source. However, since the question specifies the need for an open source solution and Cloud Spanner is not open source, the correct answer would be C, Cloud SQL for PostgreSQL with cross-region replicas, as it meets the open source requirement and can adequately serve the application's needs with some limitations.

Author: LeetQuiz Editorial Team