What should a solutions architect do to reduce costs?

Explanations:

Configuring a NAT gateway instead of NAT instances may improve reliability and scalability but will not significantly reduce data transfer costs between Amazon ECS and Amazon S3. Both NAT instances and NAT gateways incur data transfer charges when accessing the internet.

Configuring a gateway endpoint for traffic destined to Amazon S3 allows communication directly between Amazon ECS and S3 without going through the internet, thus eliminating data transfer costs associated with NAT instances or gateways for S3 traffic. This is the most cost-effective solution for reducing data transfer costs to S3.

An interface endpoint for Amazon S3 would not be appropriate, as S3 uses a gateway endpoint. Interface endpoints are used for services that are accessed through private IP addresses and do not apply to S3, making this option invalid for the scenario.

Configuring Amazon CloudFront for the S3 bucket can optimize content delivery and improve access speeds but does not directly reduce data transfer costs from ECS to S3. This involves additional costs and complexity without addressing the core issue of data transfer charges.