The correct answer is B: Modify the Auto Scaling group to use three instances across each of two Availability Zones. This setup ensures that the application is highly available by distributing instances across multiple Availability Zones, which allows the application to remain operational even if one Availability Zone experiences an outage. The other options do not achieve high availability in the same way. Option A suggests using instances across two Regions, which introduces complexity and latency issues. Option C only provides a template and does not address immediate high availability. Option D does not utilize multiple Availability Zones, which is key for high availability.