Explanation:

✅ A. Data Extensions
When working in Contact Builder, you can add data extensions to Attribute Groups. These data extensions hold the contact-related data and are then linked to the All Contacts table through sendable fields or contact keys. This allows you to build a unified contact model. Note: While you can’t “create” DEs directly inside the Attribute Group interface, you can associate existing data extensions to them, which is what the question implies.

✅ B. Relationships
Relationships are a fundamental part of Attribute Groups in Contact Builder. After adding Data Extensions, you define relationships (1-to-1 or 1-to-many) between those DEs and the Contact Record using contact keys. These relationships allow data modeling, segmentation, and personalization across related data.

❌ C. Populations
Populations are separate from Attribute Groups. You create Populations to segment large groups of contacts (like “Leads” vs “Customers”) when you have multiple root contact types. Populations exist independently in Contact Builder and aren’t created inside Attribute Groups.

❌ D. Attribute Groups
Attribute Groups are containers, not child elements. You don’t create Attribute Groups within other Attribute Groups. Instead, you create Attribute Groups as top-level groupings to logically organize related data and relationships tied to Contacts.