A solutions architect is creating an application that will handle batch processing of large amounts of data. The input data will be held in Amazon S3 and the output data will be stored in a different S3 bucket. For processing, the application will transfer the data over the network between multiple Amazon EC2 instances. What should the solutions architect do to reduce the overall data transfer costs?

Placing all EC2 instances in the same Availability Zone (AZ) is the most cost-effective solution for data transfer. Data transfer between EC2 instances within the same AZ is free. Transferring data between different AZs within the same AWS Region incurs a cost. Transferring data between different AWS Regions is significantly more expensive. Therefore, consolidating the instances within a single AZ eliminates inter-AZ data transfer costs. While an Auto Scaling group helps manage instance capacity, it doesn't directly reduce data transfer costs. Placing instances in the same AWS Region is better than different regions, but still incurs costs if instances are in different AZs within that region. Using private subnets in multiple AZs would still incur inter-AZ data transfer costs.