The most cost-effective solution for reducing storage costs while keeping infrequently accessed files available is to use an S3 Lifecycle policy. By configuring a policy to move files from S3 Standard to S3 Standard-Infrequent Access (S3 Standard-IA) after 90 days, the company can benefit from lower storage costs for older, less frequently accessed files while maintaining quick access to the more recently downloaded ones. Option D is correct because it directly addresses this requirement.