The organization policy explicitly prevents the use of service account keys, so options A and B, which involve requesting exceptions to create them, are not in line with the policy and Google's recommended practices for secure authentication.

https://console.cloud.google.com/marketplace/details/gc-launcher-for-mongodb-atlas/mongodb-atlas

https://cloud.google.com/billing/docs/how-to/export-data-bigquery "Cloud Billing export to BigQuery enables you to export detailed Google Cloud billing data (such as usage, cost estimates, and pricing data) automatically throughout the day to a BigQuery dataset that you specify." Reference: https://cloud.google.com/billing/docs/how-to/visualize-data

Billing Administrators can not create a new billing account, and the project is presumably already created.
Project Billing Manager allows you to link the created billing account to the project. It is vague on how the billing account gets created but by process of elimination

The correct answer is A because Google Cloud Billing budgets and alerts handle threshold-based notifications without custom engineering, while Cloud Billing export to BigQuery provides detailed billing data for dashboards and deeper cost analysis. This directly meets both requirements:
notifications when spend reaches thresholds and dashboard creation for detailed insights. Writing scripts to call the Cloud Billing API adds unnecessary engineering and maintenance because billing export already provides a managed data pipeline into BigQuery. Deploying Grafana on Compute Engine introduces server management and extra operational cost. Asking teams to create their own alerting stack is less controlled and less aligned with Google-recommended managed services. Budget alerts plus BigQuery export is simpler, scalable, and cost-effective.