The correct answer is A, the selection of S3 storage tiers. Amazon S3 offers different storage classes that are designed for different use cases and come with varying costs. Choosing the appropriate storage class based on how frequently the data is accessed can significantly impact the cost of storing data in Amazon S3. For example, S3 Standard is suitable for frequently accessed data, while S3 Glacier is cost-effective for long-term storage of infrequently accessed data. The other options, such as charges for transferring data into S3, bucket policies, and ingest fees, do have some cost implications, but their impact is generally less compared to the choice of storage class.