The correct answer is B, S3 Standard-Infrequent Access (S3 Standard-IA). This storage class is designed for data that is accessed less frequently but requires rapid access when needed. It offers the high durability, high throughput, and low latency of S3 Standard, with a lower per GB storage price and per GB retrieval charge. This makes S3 Standard-IA ideal for long-term storage, backups, and as a data store for disaster recovery files. The company's requirement for immediate access to rarely accessed files aligns with the benefits provided by S3 Standard-IA, making it the most cost-effective option for their use case.