To address the company's need for a cost-effective solution with no planned downtime during December for their MySQL database migration to Cloud SQL, the best approach is to use Cloud SQL's built-in features for high availability and maintenance control. Option B is correct because it directly addresses the concern of preventing maintenance during a specific period without incurring additional costs, as Cloud SQL allows users to schedule maintenance windows. Option D is also correct because creating a MySQL regional instance provides high availability by automatically failing over to a standby instance in another zone within the same region in case of an outage, ensuring no downtime. This solution is cost-effective as it leverages Google Cloud's infrastructure without the need for additional read replicas, which would increase costs. Options A and C are less optimal; opening a support ticket (A) is not a scalable or guaranteed solution, and creating read replicas (C) adds unnecessary complexity and cost for the stated requirement.