How should a customer import a file containing new and updated subscriber records that is uploaded daily?

Importing the file to a data extension using the "add and update" method is the best choice in this scenario because it allows for the efficient handling of subscriber records that require both updates to existing records and the addition of new records.

When using a data extension, it's important to note that they provide more flexibility and functionality compared to lists, especially for more complex data management scenarios. By selecting the "add and update" method, the system will check for existing records based on a unique identifier, typically the subscriber key, allowing it to either update the corresponding record with new information or add a new record if there isn't an existing match.

This method is particularly beneficial when dealing with daily uploads since it ensures that all new and updated records are processed correctly without losing any existing subscriber information. The approach effectively maintains data integrity while facilitating seamless integration of new insights or changes in subscriber data.

Other methods listed may not offer the same combination of flexibility and accuracy when trying to reconcile new and updated records effectively. For instance, importing with the "overwrite" method would replace all existing data in the data extension with the new file's contents, which could lead to loss of valuable information.