A prototype is a model or preliminary version of a system used for eliciting, analyzing, and validating stakeholder requirements. Its primary function is to provide a tangible representation for feedback before significant development resources are committed. While some advanced prototypes may have limited functionality (high-fidelity), the most fundamental and common forms, such as sketches, wireframes, and mock-ups (low-fidelity), are non-operational. They visually represent the system's structure, layout, and content without performing any actual processing or functions. Therefore, "non-operational representation" serves as the most accurate and encompassing definition that distinguishes a prototype from a partially or fully developed system.

A: A fully operational representation describes a final, production-ready system or a pilot, not a pre-development model like a prototype.

B: While some high-fidelity prototypes are partially operational, this is not a defining characteristic of all prototypes; many essential types are non-operational.

D: A flow diagram is a type of process model that depicts logic and workflow, not a visual mock-up of a system's interface or form.

1. International Institute of Business Analysis (IIBA), A Guide to the Business Analysis Body of Knowledge (BABOK Guide) v3, 2015. Section 10.35.3, "Elements," describes various forms of prototypes. It explicitly includes "Form Study Prototypes" such as wireframes and mock-ups, which are visual, non-operational models focused on the user experience. This supports the idea that a prototype can be, and often is, non-operational.

2. MIT OpenCourseWare, 6.813/6.831 User Interface Design and Implementation, Fall 2011, Lecture 5: Prototyping & Implementation. The course material distinguishes between low-fidelity and high-fidelity prototypes. Low-fidelity prototypes, such as paper sketches and storyboards, are described as non-functional tools used for quickly exploring design ideas, confirming that non-operational models are a core aspect of prototyping. (Available at: ocw.mit.edu)

3. Sommerville, I., Software Engineering, 10th Edition, 2016. Chapter 2, Section 2.3.2, "Prototyping," explains that prototypes are "early versions of software systems" used to demonstrate concepts. It discusses 'throw-away' prototypes, which are often non-operational mock-ups of the user interface, developed to understand requirements better.