The objects are “rarely accessed after 1 week” but must remain immediately available at all times. That means the storage class must support millisecond access without a restore process. S3 Standard-Infrequent Access (Standard-IA) is designed for exactly this: lower storage cost than S3 Standard, while still providing rapid access when needed.

With option B, an S3 Lifecycle rule transitions objects from S3 Standard to S3 Standard-IA after 7 days. This aligns perfectly with the usage pattern: frequent access initially, then infrequent access after a week, while keeping immediate availability.

Option C (S3 Glacier Flexible Retrieval) is not appropriate because Glacier classes are archival. Access typically requires a restore operation and retrieval time (minutes to hours), which violates “immediately available at all times.”

Option A expires objects, which deletes them after 7 days—this contradicts the requirement to keep objects available.

Option D is irrelevant because delete markers exist only when versioning is enabled. The bucket does not have versioning enabled, so this rule would not help.

Therefore, transitioning to S3 Standard-IA after 7 days is the correct cost-optimized solution while maintaining immediate availability.