The correct answer is D. Stackdriver Monitoring (now Google Cloud Monitoring) is designed to monitor resources and set up alerts with minimal development effort. It provides pre-built metrics, dashboards, and alerting policies that can be configured via the UI or API without writing custom code. Cloud Function (A) would require developing custom monitoring logic, which increases development time. Cloud Pub/Sub (B) is a messaging service, not a monitoring tool. Stackdriver Error Reporting (C) focuses on application errors, not general service degradation metrics like latency or uptime. Thus, D directly addresses the requirement to minimize development time while ensuring effective monitoring and alerting.