Northern Trail Outfitters (NTO) has been storing web behavior in a data extension for several...

Comprehensive Explanation:

In Salesforce Marketing Cloud, applying a Retention Policy to a data extension helps manage data storage by automatically deleting records after a specified period.​

Option B:By deleting data older than twelve months, NTO ensures that only relevant data remains. Subsequently, configuring a Retention Policy via Contact Builder will automate the deletion of records beyond the desired retention period moving forward.​

Option D:Creating a new data extension with a predefined Retention Policy is a proactive approach. This ensures that only data within the desired retention period is stored, and it avoids potential issues with applying retention policies to existing data extensions that have exceeded certain thresholds.​