The correct answer is D because gcloud deployment-manager deployments update is the command used to update an existing deployment with minimal downtime. This command allows you to add or remove resources and update the properties of existing resources within a deployment.

• Option A is incorrect as gcloud deployment-manager resources update is not a valid command for updating deployments.
• Option B is incorrect because gcloud deployment-manager deployments create is used to create a new deployment, not update an existing one.
• Option C is incorrect as gcloud deployment-manager resources create is not applicable for updating deployments.

For more details, refer to the Google Cloud Deployment Manager documentation.