Final answer:
For a subscriber with multiple values for an attribute, Data Extensions should be used as they offer a relational database structure suitable for handling complex data scenarios.
Step-by-step explanation:
If a subscriber has more than one value for an attribute, the storage model that should be used is Data Extensions. Data Extensions in many marketing platforms allow for a more flexible and relational database structure. They can store multiple values for a single subscriber by utilizing tables that contain rows and columns, much like a traditional database. This enables one to uniformly handle more complex data scenarios where subscribers might have multiple entries for a single attribute, such as multiple phone numbers or addresses.
While Profile Attributes are designed to store a single piece of information per subscriber, Publication Lists are used for managing subscription preferences, and Lists are basic storage that often have a flat structure and are not optimal for multiple values per attribute, making Data Extensions the most suitable choice for this scenario.