Answer: `gcloud deployment-manager deployments update`

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](https://cloud.google.com/deployment-manager/docs).

Author: LeetQuiz Editorial Team