To build an automated pipeline that deploys the application when the image is updated in Google Container Registry (GCR) with minimal development effort, the best approach is to use Cloud Build to trigger a Spinnaker pipeline. Cloud Build can be configured to automatically trigger builds and deployments upon changes to the container registry. Spinnaker is a continuous delivery platform that supports deploying to multiple cloud providers, including Google Cloud, and can be easily integrated with Cloud Build for automated deployments. This approach minimizes development effort by leveraging existing Google Cloud services and their integrations. Option B, using Cloud Pub/Sub to trigger a Spinnaker pipeline, is also a viable solution as Cloud Pub/Sub can be used to listen for changes in GCR and trigger the Spinnaker pipeline accordingly. However, Cloud Build provides a more direct and simpler integration for this specific use case. Options C and D involve more custom solutions (Jenkins pipeline and custom deployment service) which would require additional development effort and are not as streamlined as using Cloud Build with Spinnaker.