Object Versioning in Oracle Cloud Infrastructure (OCI) Object Storage is a feature that is configured at the bucket level, not at the namespace level. A namespace is a logical container for all buckets within a tenancy, but granular features like versioning, logging, and lifecycle policies are managed on a per-bucket basis. This allows administrators to apply different data protection strategies to different buckets based on their specific requirements. Enabling versioning at the namespace level would incorrectly apply the setting to all buckets in the tenancy, which is not how the service is designed.

A. Retention rules are the specific OCI feature that provides WORM (Write-Once-Read-Many) compliance by making objects immutable for a defined period, preventing deletion or modification.

B. OCI Identity and Access Management (IAM) policies can be configured to grant users or groups in one tenancy access to Object Storage resources, such as buckets, in another tenancy.

C. Object Lifecycle Management policies are used to define rules that automatically transition objects to different storage tiers (e.g., Archive) or delete them based on their age.

1. Oracle Cloud Infrastructure Documentation, "Using Object Versioning". This document states: "You enable Object Versioning at the bucket level. You can't enable versioning at the namespace level." This directly supports why option D is the correct answer.

2. Oracle Cloud Infrastructure Documentation, "Using Retention Rules". This document states: "Retention rules provide an immutable storage option for data stored in Object Storage... When an object is under a retention rule, the object cannot be modified, overwritten, or deleted." This confirms that statement A is true.

3. Oracle Cloud Infrastructure Documentation, "Using Object Lifecycle Management". This document states: "Use lifecycle policy rules to automatically archive or delete objects after a specified amount of time." This confirms that statement C is true.

4. Oracle Cloud Infrastructure Documentation, "Accessing Object Storage Resources in Other Tenancies". This document explains the process: "You can grant a group in your tenancy (Tenancy A) access to a bucket or an object in another tenancy (Tenancy B)." This confirms that statement B is true.