The correct answer is A. Developing a Cloud Monitoring dashboard based on the BigQuery metric slots/allocated_for_project allows each team to monitor the number of slots allocated to their project and their current usage, providing clear visibility into their slot utilization. This method is efficient and directly addresses the need for slot usage monitoring.

Option B is incorrect as it focuses on monitoring data volume rather than slot usage. Option C, while feasible, is inefficient due to the need for creating custom metrics for each project, potentially leading to a cumbersome number of metrics. Option D is also inefficient, as aggregating logs at the organization level can complicate the separate monitoring of slot usage for individual projects. Reference: BigQuery Reservations Monitoring