During deployment, old and new versions briefly run simultaneously. A database migration adds a required column. New code writes it; old code doesn't. What happens?

1. A Execute the database schema migration and the application code deployment as a single, atomic operation to ensure that no instance ever encounters a schema mismatch.
2. B Temporarily stop the application traffic to run the migration through a maintenance window, ensuring that the database state is fully settled before restarting the cluster.
3. C Rely on the database's default value mechanism to automatically populate the new column for all writes originating from older application instances during the rollout period.
4. D Utilize the 'Expand-Contract' pattern: first add the column as nullable, deploy code that writes to it, backfill the data, and finally apply the strict Not-Null constraint.