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The approach of adding all of the keys into a formatted file and using the command to update project metadata is the most efficient method for deploying SSH keys across multiple instances. When you use the `gcloud compute project-info add-metadata` command, it allows you to set metadata at the project level, which applies to all instances within that project. This means that you can manage your SSH keys centrally and deploy them to all relevant instances with just a single command application.

This method significantly reduces the steps and time involved in the process. Once the keys are in the project metadata, they automatically propagate to all instances that are configured to use SSH keys from project metadata. Consequently, the team can access all instances securely without having to manually add keys to each individual instance.

Selecting commands that apply to individual instances, such as adding metadata to each instance individually, would require more steps and lead to a more complex and error-prone process. Additionally, commands like `gcloud compute ssh` are designed for establishing SSH connections rather than managing SSH keys. Using the user interface to upload keys one at a time would also be time-consuming and inefficient compared to the centralized method of managing keys at the project level.