What tool should Northern Trail Outfitters use to ensure that subscribers opt out at the list level?

The correct tool for ensuring that subscribers opt out at the list level is the Publication Lists. This type of list is specifically designed for managing subscription preferences and allows subscribers to opt out at a more granular level. When a subscriber opts out from a Publication List, they will no longer receive emails associated with that specific list while remaining subscribed to other lists. This is particularly useful for organizations that send different types of content to their subscribers and want to respect their preferences regarding specific communications.

In contrast, the other options do not provide the same level of granularity in managing subscriber preferences. For example, Journey Lists are focused on managing subscribers within a specific Journey and do not inherently handle opt-out preferences at the list level. Email Lists and Data Lists are more general and are typically used for sending emails but do not offer the specific functionality for managing subscription preferences as well as Publication Lists do.