For sending a promotional email to one million subscribers, a Data Extension is the most suitable data structure. Data Extensions offer robust data management capabilities, scalability, and flexibility for handling large volumes of data and complex segmentation, which is essential when dealing with a large audience size like one million subscribers. This setup allows for more detailed targeting and personalization strategies.
Reference: Salesforce Help - Data Extensions

To manage the operations of all North American Business Units in Marketing Cloud, including Email Studio, Mobile Connect, and Social Studio, the two roles suitable for the new hire are:
C). Marketing Cloud Regional or Local Administrator: This role provides administrative capabilities specific to regional operations, suitable for managing multiple business units within a geographic area.
D). Marketing Cloud Administrator: This standard role provides full administrative rights across the entire Marketing Cloud environment, ideal for overseeing operations across various channels and business units.
These roles ensure the new hire has the necessary permissions to effectively manage and oversee the various platforms and business units within North America.
Reference: Salesforce Help - User Roles

Smart Capture is the feature that should be used to create a survey form. Smart Capture is a feature that allows marketers to create and insert forms within landing pages or emails, such as surveys, polls, or feedback requests. Smart Capture can be customized with different fields, labels, buttons, and actions. Smart Capture can capture subscriber responses and store them in a data extension or update existing records in Marketing Cloud.