You need to alert on-call engineers about a production service degradation with minimal development work. Which service should you use to monitor resources and send alerts?

Stackdriver Monitoring (now Google Cloud Monitoring) is the correct choice because it is specifically designed for monitoring resources and sending alerts based on predefined metrics and thresholds. It offers out-of-the-box integrations with Google Cloud services, allowing for minimal development work to set up alerts for service degradation. Cloud Functions can be used for custom monitoring logic, but it requires more development effort. Cloud Pub/Sub is a messaging service, not a monitoring solution. Stackdriver Error Reporting focuses on capturing and grouping application errors, not general resource monitoring for degradation.