Which Multi-Tenancy model offers the highest security/isolation but the lowest resource efficiency (highest cost)?

1. A Discriminator Column (Shared Table), where a single `tenant_id` column is used to filter all data at the application or query layer.
2. B Schema-per-Tenant (Shared Database), where each customer is assigned a separate, logical namespace but shares the same underlying physical instance.
3. C Logical Sharding, where tenant data is automatically partitioned across several nodes based on a high-cardinality hashing algorithm to ensure uniform load.
4. D Database-per-Tenant (Silo), where each individual client is provisioned with their own dedicated and physically isolated database instance.