The correct answer is B 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, update properties of existing resources, or a combination of both in a single request.

• Option A is incorrect as gcloud deployment-manager deployments create is used for creating a new deployment, not updating an existing one.
• Options C & D are incorrect because resources is not a valid parameter for updating deployments in the context of the gcloud command-line tool.

For more detailed steps on updating deployments, refer to the GCP documentation on Deployment Manager Update Deployment.