To optimize platform costs for hundreds of Cloud SQL for MySQL instances following Google-recommended practices, the best approach involves identifying and addressing inefficiencies. Option A, using Query Insights, is a powerful tool for identifying idle instances which can then be decommissioned or downsized to save costs. Option C, running the Recommender API, is another Google-recommended practice to identify overprovisioned instances that can be resized to match actual usage, thus reducing costs. While options B and D are good database management practices, they are not directly related to optimizing platform costs as effectively as A and C.