Which solution will meet these requirements with the LEAST operational effort?

Explanations:

Using multiple AWS Regions introduces latency and complexity, along with potential issues with consistency. Aurora PostgreSQL Cross-Region Replication is not the most efficient solution for high availability within a single application architecture. Additionally, it increases operational effort due to the need for managing resources across regions.

Configuring the Auto Scaling group across multiple Availability Zones and setting up the database as Multi-AZ ensures high availability and automatic failover, minimizing downtime and data loss. Using RDS Proxy provides connection management and improves application availability, reducing operational effort significantly.

Operating in a single Availability Zone does not meet high availability requirements. Generating hourly snapshots does not provide real-time recovery and can lead to data loss during a failure. This method increases downtime and is not suitable for a business-critical application requiring minimal disruption.

Configuring the Auto Scaling group across multiple AWS Regions complicates the architecture and adds latency. Writing data to S3 and using Lambda introduces potential data consistency issues and more operational overhead, detracting from the goal of minimizing effort while ensuring high availability.