To migrate a MySQL database to Google Cloud with minimal downtime and following Google-recommended practices, the best approach is to use Cloud SQL, which is a fully managed database service. Option B suggests using the mysqldump utility to take a backup of the existing on-premises database and then importing it into Cloud SQL. This method is straightforward but may involve some downtime during the import process. Option D, however, suggests creating an external replica and using Cloud SQL to synchronize the data to the replica. This method is more aligned with Google's recommended practices for minimal downtime migrations, as it allows for continuous synchronization of data from the on-premises database to Cloud SQL, significantly reducing downtime. Therefore, the correct answer is D.