Which action is necessary to illustrate decision points in a flowchart?

In flowcharting, decision points are critical components that indicate where a choice must be made, affecting the subsequent path of the process. The standard convention for representing these decision points is the use of diamonds. This geometric shape visually conveys that a decision must be made, leading to different outcomes and paths in the flowchart based on the answer to the question posed at that point.

Using rectangles, while essential for other functions in flowcharts—such as representing processes or actions—does not convey the idea of a decision point. Circles are typically used for terminators or to show connections within the flowchart, which do not relate to decision making. Adding notes can provide additional context or explanations but does not visually represent any decision-making process or point within the flowchart itself. Therefore, incorporating diamonds is the established and widely accepted method for illustrating decision points effectively in a flowchart.