Explanation
Transactional email send with the event notification service response can be used to send password reset emails and get real-time feedback on the delivery status. If the email is not sent, an API call can be triggered to send an SMS message instead. References:
https://help.salesforce.com/articleView?id=sf.mc_es_event_notification_service.htm&type=5
https://developer.salesforce.com/docs/atlas.en-us.noversion.mc-apis.meta/mc-apis/messageDefinitionSends.htm

Explanation
Three questions that are relevant to determining a solution for automating a process of importing data from a data warehouse into Marketing Cloud are:
Is the data file a delta or a historical file? This question will help determine how to handle existing records and avoid duplication or overwrite. A delta file contains only new or updated records, while a historical file contains all records regardless of changes.
Will the data file be placed on the SFTP at the same time daily? This question will help determine how to schedule an automation or trigger an event based on file drop. If the data file is placed on the SFTP at different times, then a File Drop Automation may be more suitable than a Scheduled Automation.
Does someone need to be notified if an error happens on import? This question will help determine how to handle errors and exceptions during import. If someone needs to be notified, then an email notification activity or a verification activity may be added to the automation.
Whether the data extension has a data relationship or not is not relevant to determining a solution for importing data, as it is only used for creating relationships between data extensions in Email Studio. Whether the file has more than 5,000 rows or not is not relevant to determining a solution for importing data, as it does not affect how data is imported or processed. References:
https://help.salesforce.com/s/articleView?id=sf.mc_as_file_drop_automation_studio_triggers.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.mc_as_import_file_activity.htm&type=5

Explanation
To import new contacts gathered from an external web form at high frequency into Marketing Cloud, Northern Trail Outfitters should perform a daily bulk export to SFTP for import via automation. This will allow them to batch the contacts into one file per day and import them into Marketing Cloud using an automation that runs at a scheduled time. References:
https://help.salesforce.com/s/articleView?id=sf.mc_as_import_activity.htm&type=5