A company recently experienced a significant data loss when proprietary Information was leaked to a competitor. The company took special precautions by using proper labels; however, email filter logs do not have any record of the incident. An Investigation confirmed the corporate network was not breached, but documents were downloaded from an employee's COPE tablet and passed to the competitor via cloud storage. Which of the following is the BEST remediation for this data leak?

User training

CASB

MDM

DLP