The question asks for an SQL database architecture that minimizes costs and user disruption during a gradual rollout, with the ability to scale up or down based on demand, such as nights and holidays. Option A suggests using Oracle Real Application Cluster (RAC) databases on Bare Metal Solution for Oracle, which is a high-cost solution and not the best fit for minimizing costs or for a cloud-native approach. Option B proposes using sharded Cloud SQL instances, which allows for isolating stores to individual instances, minimizing disruption during rollout, and enabling cost-effective scaling. However, managing multiple instances can become complex. Option C suggests using a Bigtable cluster with autoscaling, but Bigtable is a NoSQL database, not SQL, which doesn't meet the requirement for an SQL database. Option D recommends using Cloud Spanner with a custom autoscaling solution. Cloud Spanner is a fully managed, scalable, relational database service that supports SQL queries, and adding a custom autoscaling solution can help manage costs and performance during varying loads, making it the most suitable option among the choices provided.