Correct answer is C. Client-side encryption with Azure Key Vault. This method is not supported for encrypting data at rest in an Azure Databricks workspace. Here's why the other options are supported:

• A. Server-side encryption with customer-managed keys in Azure Blob Storage: Azure Blob Storage supports server-side encryption where data is automatically encrypted before being written to disk and decrypted when read.
• B. Encryption using Azure Managed Disk Encryption: This method helps protect data on Azure managed disks by encrypting the disk volumes.
• D. Transparent Data Encryption (TDE): TDE encrypts the storage of an entire database by using a symmetric key called the database encryption key, protecting the data stored in the database files.