The primary tool used in structured design is a:

Answer: Option A

structure chart: This is the correct answer. In structured design, the primary tool used is a structure chart. A structure chart is a visual representation of the modules or components of a system and their relationships, showing the hierarchy and connections between them.

data-flow diagram: While data-flow diagrams are important in structured design, the primary tool is the structure chart.

program flowchart: Program flowcharts are used to represent the flow of control in a program but are not the primary tool in structured design.

module: A module is a component of a system, and while important in structured design, the primary tool is the structure chart.

Therefore, in the context of the question, the primary tool used in structured design is a structure chart (Option A).