The Infrequent Access tier is the most suitable choice as it is specifically designed for data that is accessed infrequently but must be available immediately upon request. This tier offers a lower storage cost compared to the Standard tier, fulfilling the cost-minimization requirement. Crucially, it has a minimum retention requirement of 31 days, which perfectly aligns with the specified backup retention period. Data retrieval is immediate, satisfying the accessibility constraint. This combination of lower cost, immediate access, and a matching 31-day minimum retention period makes it the optimal solution for this backup scenario.

A. Archive tier: This tier is incorrect because it does not provide immediate accessibility; data retrieval can take several hours. It also has a 90-day minimum retention period, which would incur cost penalties for a 31-day retention plan.

B. Auto-Tiering tier: While providing immediate access, Auto-Tiering is a feature for optimizing costs for data with unpredictable or changing access patterns. For a predictable workload like backups (infrequently accessed), directly selecting the Infrequent Access tier is more direct and cost-effective.

C. Standard tier: This tier provides immediate access but has the highest storage cost of the "hot" tiers. It does not meet the primary requirement to minimize cost, as a cheaper and equally suitable option (Infrequent Access) is available.

1. Oracle Cloud Infrastructure Documentation - Object Storage Tiers:

Source: Oracle Cloud Infrastructure Documentation, "Overview of Object Storage," section "Object Storage Tiers."

Details: This document explicitly states, "Use the Infrequent Access tier for data that you access infrequently, but that must be available immediately when needed... The minimum retention requirement for objects in the Infrequent Access tier is 31 days." This directly supports the choice of option D. It also describes the Archive tier's retrieval time as "Time to First Byte (TTFB) of 1 hour" and a "minimum retention requirement... of 90 days," invalidating option A. It positions the Standard tier for "data that requires fast, immediate, and frequent access," making it more expensive and thus invalidating option C.

2. Oracle Cloud Infrastructure Documentation - Auto-Tiering:

Source: Oracle Cloud Infrastructure Documentation, "Using Auto Tiering."

Details: The documentation explains that "Auto Tiering monitors data access patterns and automatically moves objects... to the most cost-effective access tier." This is ideal for mixed or unknown workloads, not for a predictable backup workload where the Infrequent Access tier is the known, cost-effective choice from the start, making option B less optimal than D.