What is defined as the model that explains how the product works?

The design model is the correct answer because it serves as a framework that outlines how a product operates from a user's perspective. It encapsulates the principles, behaviors, and functionalities of the product, ensuring that users can understand how to interact with it effectively. The design model provides clarity on the relationships between various elements of the product, allowing both designers and users to comprehend the system's logic and flow.

In the context of UX design, the design model is crucial because it directly informs the user interface and overall user experience. By clearly defining how the product works, designers can create intuitive interfaces that align with users' mental models, thereby enhancing usability. Understanding the design model helps ensure that every part of the product contributes cohesively to the user's interaction with it.

The other options relate to different aspects of a product's development and interaction but do not encapsulate the explanatory framework in the same way. User interfaces are about how users interact with the product visually and functionally, system architecture focuses on the technological structure of the application, and user experience encompasses the overall feelings and perceptions users have when interacting with the product. While they are all important components of product design, they do not specifically serve as models that explain operational mechanics like the design model does.