Structured methodologies use which of the following to illustrate each level of a system's design?

Structured methodologies often utilize structure charts to illustrate each level of a system's design. Structure charts provide a visual representation of the system's components and the relationships between them, breaking down complex systems into manageable parts. They help in depicting how modules or functions within the system interact with each other, making it easier to understand the overall architecture of the system.

By organizing functions in a hierarchical manner, structure charts enable developers and analysts to see the structure and flow of the system more clearly, facilitating better design and implementation processes. This method aligns well with the goals of structured methodologies, which aim to promote clarity and organized design in system development.

Other options, such as Gantt charts, are more focused on project scheduling and task timelines rather than the intrinsic design structure of systems. Process specifications detail the specific functionalities of each process but do not visually represent the structural relationships. Data flow diagrams illustrate the flow of data within a system but do not specifically represent the hierarchy and structure of the system's components as effectively as structure charts do.