Which feature is used for full disk encryption on Windows systems?

BitLocker is the feature used for full disk encryption on Windows systems. It is designed to provide a high level of security by encrypting the entire disk volume, ensuring that the data is not accessible without proper authentication. This means that if the physical hard drive is removed from the system, the data on it remains secure, as it cannot be read without the appropriate keys or passwords.

BitLocker utilizes Trusted Platform Module (TPM) technology to enhance security by storing encryption keys in a way that they are tied to the specific computer hardware. This reduces the risk of unauthorized access and helps protect sensitive information against theft or loss.

The other options serve different purposes: Encrypting File System (EFS) is used for encrypting individual files rather than entire disks, a VPN is used for creating secure network connections over the internet, and Windows Defender offers antivirus protection rather than encryption features. Thus, BitLocker is the correct choice for full disk encryption.