To minimize downtime for migrating a highly transactional application with a 500 GB MySQL database instance to Cloud SQL, the best approach involves using the Database Migration Service with a Continuous migration job type. This method allows for the initial full dump of the databases followed by continuous replication of changes (CDC mode), ensuring minimal downtime. Option B outlines this process correctly, including the steps to wait for the replication delay to minimize before promoting the new Cloud SQL instance and updating application connections. Option A suggests using Datastream, which is not the most efficient method for minimizing downtime in this scenario. Option C proposes a One-time migration job, which would require a maintenance window and stopping write workloads, leading to more downtime. Option D involves manual steps with mysqldump, which is not scalable or efficient for a 500 GB instance and would result in significant downtime.