What is created to validate a system design without including aspects related to appearance or materials?

The correct choice is to create a Proof of Principle or Concept. This type of document or model focuses on demonstrating that certain concepts or theories can be successfully applied within the constraints of the system design. It essentially validates whether a certain functionality works as intended and can fulfill the required operational parameters without delving into aesthetic aspects or material specifications. This makes it an effective tool for assessing the feasibility of ideas before moving into more detailed stages of design or prototyping.

On the other hand, other options address different aspects of the design process. A prototype often includes physical elements and represents a more advanced stage in modeling, which includes considerations of materials and user interface design. A design mockup similarly aims to represent both look and feel of a product but does not focus solely on the functional validation of theoretical concepts. Proof of failure would generally not be constructed to validate system design; instead, it would highlight shortcomings or issues, which does not align with the goal of confirming theoretical feasibility.