You should use Cloud Monitoring and utilization metrics. -> Correct. Cloud Monitoring can provide a wide array of metrics including query execution times, storage utilization, and costs which are vital for understanding how much you are using BigQuery. Utilization metrics specifically can help you understand how much of your provisioned capacity is being used, which can help you make a more informed decision about whether Flat-rate billing would be cost-effective.
You should use Cloud Monitoring and CPU utilization metrics. -> Incorrect. Cloud Monitoring is correct, but focusing solely on CPU utilization metrics would not provide a complete picture of BigQuery usage. BigQuery is a serverless, highly scalable, and cost-effective multi-cloud data warehouse, and its costs are more tied to the queries run and the data stored rather than CPU utilization.
You should use Cloud Logging and audit log data. -> Incorrect. While Cloud Logging and audit log data can provide insights into who is accessing BigQuery and what operations are being performed, it won't necessarily help you understand the resource utilization in a way that would inform a decision about billing models.
You should use Cloud Logging and CPU utilization metrics. -> Incorrect. Cloud Logging is not the best tool for this particular task. CPU utilization metrics, as mentioned before, do not give a complete picture of resource usage for BigQuery.
https://cloud.google.com/bigquery/docs/monitoring