You need to protect sensitive files from unauthorized users even if the disk is stolen. What feature should you use and on what file system?

EFS on NTFS

Explanation:

EFS (Encryption File System) allows users to store confidential information about a computer when people who have physical access to your computer could otherwise compromise that information, intentionally or unintentionally. EFS is especially useful for securing sensitive data on portable computers or on computers shared by several users.  An attacker can also steal a computer, remove the hard drive(s), place the drive(s) in another system, and gain access to the stored files. Files encrypted by EFS, however, appear as unintelligible characters when the attacker does not have the decryption key.

The Encrypting File System (EFS) that is included with the operating systems provides the core file encryption technology to store NTFS files encrypted on disk.