Which feature provides secure encryption of all user data stored on a Windows Phone 8.1 device?

The correct choice highlights the capability of BitLocker to provide secure encryption for user data on a Windows Phone 8.1 device. BitLocker is a full-disk encryption feature available in Windows operating systems, designed to protect data by providing encryption for entire volumes. On mobile platforms like Windows Phone, it ensures that all user data stored on the device, including applications and system files, is encrypted, making unauthorized access significantly more difficult.

With BitLocker, if the device is lost or stolen, the encrypted data remains protected as long as the encryption key is not compromised. This is crucial for ensuring user confidentiality and data integrity, particularly in contexts where sensitive information might be involved.

While other security features mentioned, such as Device Guard, Windows Information Protection, and Secure Boot, also contribute to the overall security ecosystem of a device, they do not specifically provide encryption for user data in the same manner as BitLocker. Device Guard focuses on application security by ensuring that only trusted applications can run, Windows Information Protection manages how apps handle enterprise data, and Secure Boot helps prevent unauthorized firmware and software from running during the boot process. Each has its unique role, but for the purpose of data encryption on Windows Phone 8.1, BitLocker is the appropriate feature.