Correct Option:

D. Compile all keys into a properly formatted file and use the gcloud compute project-info add-metadata command to upload them to the project's metadata: This approach is correct because it applies the SSH keys to all instances within the project, including the Bastion host, in one efficient step.

Incorrect Options:

A. Format the keys as required and manually upload each key one by one through the user interface: This method is inefficient and time-consuming, as it requires manual uploads for each key.

B. Use the gcloud compute ssh command to upload all the keys at once: This is incorrect because the gcloud compute ssh command is designed for SSH access, not for uploading SSH keys.

C. Compile all keys into a properly formatted file and use the gcloud compute instances add-metadata command to upload them to each instance individually: While this method works, it is less efficient than applying the keys at the project level, as it requires repeating the process for each instance.