The gcloud storage rsync command is the most efficient tool for this task. It synchronizes the contents of a source directory with a destination, making them identical. This command is highly efficient because it performs parallel copies, retries on failures, and, most importantly, only copies objects that do not exist at the destination or have changed since the last sync. This checksum-based comparison avoids redundant data transfer, making it ideal for large-scale or resumable migration scripts as specified in the scenario.

A. gcloud storage buckets copy ... is an invalid command. The gcloud storage buckets command group is used for managing bucket-level properties (e.g., creating, updating), not for copying objects.

B. gcloud storage cp ... would require a recursive flag (-r) to copy a directory's contents. Even with the flag, it is less efficient than rsync because it does not check if the object already exists and is unchanged at the destination.

C. gsutil mv ... moves the objects, which means it copies them to the destination and then deletes them from the source. The requirement is to copy, not move.

1. gcloud storage rsync Documentation: "The rsync command makes the contents of a destination URI match the contents of a source URI... It is the most efficient way to synchronize data between locations because it only copies missing or outdated files and can perform parallel transfers."

Source: Google Cloud SDK Documentation, gcloud storage rsync command reference.

2. gcloud storage cp Documentation: "The cp command allows you to copy data between your local file system and the cloud, within the cloud, and between cloud storage providers... For recursive copies of directories, use the -r flag." This documentation does not mention the differential synchronization feature that makes rsync more efficient for this use case.

Source: Google Cloud SDK Documentation, gcloud storage cp command reference.

3. gsutil mv Documentation: "The gsutil mv command allows you to move data... This is equivalent to running gsutil cp -r on the source and destination, then gsutil rm -r on the source." This confirms the command deletes the source, which is not what the question asks for.

Source: Google Cloud SDK Documentation, gsutil mv command reference.

4. gcloud storage buckets Documentation: "The gcloud storage buckets command group lets you create and manage Cloud Storage buckets." This confirms the command is for bucket management, not object operations.

Source: Google Cloud SDK Documentation, gcloud storage buckets command group reference.