What is the MOST cost-effective action that should be taken to avoid downtime?

Explanations:

Migrating to Amazon DynamoDB would not address the issue of downtime due to AWS operating system patches, as it is a different database service and not a solution for reducing downtime in Amazon RDS for MySQL.

Enabling cross-Region read replicas does not help with minimizing downtime during maintenance patches. Traffic would still be directed to the primary RDS instance during the patching, resulting in downtime.

Enabling a read replica and directing traffic to it would not fully eliminate downtime during AWS OS patching, as read replicas are still dependent on the primary instance for certain functions.

Enabling an Amazon RDS for MySQL Multi-AZ configuration provides high availability by maintaining a synchronous standby in another Availability Zone. This minimizes downtime during patching and maintenance.