Explanation

Amazon DynamoDB is a fully managed NoSQL database service that provides:

• Automatic scalability: DynamoDB automatically scales up and down based on workload demands without manual intervention
• Built-in high availability and failover protection: DynamoDB automatically replicates data across multiple Availability Zones and provides automatic failover
• Automated backups: DynamoDB offers point-in-time recovery and on-demand backup capabilities
• Operational efficiency: As a fully managed service, DynamoDB eliminates the operational overhead of managing EC2 instances, patching, scaling, and backup management

Why other options are less efficient:

• Option A: Adding EC2 instances requires manual scaling management and doesn't provide automatic failover protection
• Option B: Running a copy in another AZ provides some failover protection but still requires manual management of scaling, backups, and failover coordination
• Option D: Migrating to a relational database would require significant schema changes and may not be suitable for key-value NoSQL workloads

DynamoDB is specifically designed for key-value NoSQL workloads and provides all the required features as a managed service, making it the most operationally efficient solution.