The AI assistant agrees with the suggested answer, Option A.

Reason for choosing this answer:
The question requires a solution that provides both high availability (HA) and low latency for read and write operations across multiple US regions. Option A addresses these requirements optimally based on standard Google Cloud SQL best practices.

High Availability (HA): Deploying 2 Cloud SQL instances in the `us-central1` region with HA enabled typically refers to setting up a primary instance and a standby instance in different zones within the `us-central1` region. This configuration ensures automatic failover in case of a zone outage or primary instance failure, meeting the HA requirement for the database and minimizing downtime for write operations and local reads.
Low Latency for Read Operations: Creating read replicas in `us-east1` and `us-west1` directly addresses the need for low latency for applications located in those respective regions. Applications can connect to the closest read replica, significantly reducing network latency for read queries. This also offloads read traffic from the primary instance, improving overall performance.
Low Latency for Write Operations: Write operations will primarily occur on the HA-enabled primary instance in `us-central1`. For applications located in `us-central1`, writes will have low latency. While applications in `us-east1` and `us-west1` will experience slightly higher latency for writes due to cross-regional network travel, this is the standard trade-off for a single-primary, multi-region read replica architecture, which is generally efficient and scalable for most use cases like the one described (100 databases, 8000 tables total). Cloud SQL's architecture does not natively support multi-region, active-active write distribution for a single database instance without significantly increasing complexity or potential for write conflicts/latency.
Efficiency and Scalability: This setup is a common and efficient pattern for distributing reads globally while maintaining a strong consistency model (via a single primary for writes) and high availability for the primary. The scale of 100 databases with 80 tables each (total 8000 tables) is well within the capabilities of a single HA Cloud SQL for PostgreSQL instance with regional read replicas.

Reasons for not choosing the other answers:

Option B: Deploy 2 Cloud SQL instances in the `us-central1` region, and create read replicas in `us-east1` and `us-west1`.
- This option is incorrect because it explicitly omits "with HA enabled." High availability is a core requirement mentioned in the question, making this option unsuitable.
Option C: Deploy 4 Cloud SQL instances in the `us-central1` region with HA enabled, and create read replicas in `us-central1`, `us-east1`, and `us-west1`.
- The phrase "Deploy 4 Cloud SQL instances in the `us-central1` region with HA enabled" is problematic and not a standard or efficient Cloud SQL HA configuration for a single logical database. Cloud SQL HA typically involves one primary and one standby instance (total 2 instances) for regional HA. Deploying "4 instances" in the primary region with HA enabled is an unusual and likely unnecessary over-provisioning or misinterpretation of Cloud SQL's HA model, which does not support multiple active primaries for the same logical instance.
- Creating read replicas in the same region as the primary (`us-central1`) is generally redundant if the goal is to serve applications in geographically diverse regions and if the primary itself is already serving local traffic with low latency. While technically possible, it adds unnecessary complexity or cost without significantly improving multi-regional read latency compared to serving reads from the primary itself or relying on read replicas in other regions.
- This option suggests an "overkill" and less efficient setup compared to the simpler and more standard Option A.
Option D: Deploy 4 Cloud SQL instances in the `us-central1` region, and create read replicas in `us-central1`, `us-east1` and `us-west1`.
- Similar to Option B, this option is incorrect because it does not include "with HA enabled," which is a critical requirement.
- Similar to Option C, deploying "4 Cloud SQL instances" in the primary region is an unusual and inefficient configuration.

Citations:

Cloud SQL High Availability
https://cloud.google.com/sql/docs/mysql/high-availability (Note: While the URL is for MySQL, the HA principles for Cloud SQL are generally consistent across database engines like PostgreSQL, ensuring a primary and standby instance in different zones.)
Cloud SQL Overview of Read Replicas
https://cloud.google.com/sql/docs/mysql/read-replicas (Note: Similar to HA, the concept and benefits of read replicas are consistent across Cloud SQL database engines.)
Managing high availability (Cloud SQL for PostgreSQL)
https://cloud.google.com/sql/docs/postgres/configure-ha
About read replicas (Cloud SQL for PostgreSQL)
https://cloud.google.com/sql/docs/postgres/read-replicas