To ensure an SSH key works on every instance in the project efficiently, uploading the public key to project metadata (A) is the correct approach. Project metadata applies to all instances in the project, ensuring new instances automatically include the key. Existing instances will inherit the key after a restart. While option D (gcloud compute ssh) adds the key to individual instance metadata when used, it requires manual or scripted execution for each instance, making it less efficient. Options B (per-instance metadata) and C (custom image) are not scalable or efficient for all instances.