Which feature on Windows Phone 8.1 provides secure encryption of all user data stored locally on
internal data partitions?

A.
Secure Boot

B.
Encrypted File System

C.
BitLocker

D.
TrueCrypt

Explanation:
The Windows Phone OS supports using BitLocker technology to encrypt all user data stored locally
on internal data partitions.
Reference: Secure boot and device encryption overview
https://dev.windowsphone.com/en-US/OEM/docs/Phone_BringUp/Secure_boot_and_device_encryption_overview