Implementing geofencing within the data management policies is the most effective way to ensure that data does not leave the European Union, as it provides controls that can enforce the physical boundaries set by the regulations. Geofencing uses GPS or RFID-enabled software to trigger a pre-programmed action when a mobile device enters or exits a virtual boundary set up around a geographical location, in this case, the EU. Other options like redundant data storage or encryption do not inherently restrict the transfer of data across geographical lines. Data loss prevention focuses on monitoring and protecting data in use, in motion, and at rest, but does not pertain specifically to geographical restrictions.