The Google-recommended and most automated solution for tracking spending against a specific limit for a group of projects is using Cloud Billing Budgets.
* Structure: Placing all sandbox projects under a single Folder allows the budget to be scoped accurately to the entire sandbox environment's spend.
* Automation/Notification: A Budget Alert is a native feature that automates the monitoring and notification process. You can configure multiple alert thresholds (e.g., 50%, 90%, 100%) and specify email addresses for recipients, including the engineers (users) and administrators.
* Cost Minimization: This uses native Google Cloud functionality, minimizing the need for custom code (Option D) or additional external services (Option A and D). Option C imposes a hard stop, which is not what the question asks for (which is notification).
Reference: Google Cloud Billing documentation, specifically the section on "Setting and managing budgets and budget alerts." "You can scope a budget to an entire Cloud Billing account, or to a subset of all or some of the account's projects, folders, or organization."