Final answer:
To select stars in a single step and change the fill color to C=100, M=80, Y=25, the Magic Wand Tool should be used. The Magic Wand Tool is designed for selecting similar objects on a graphic design platform. Other tools like the Direct Selection Tool or Eyedropper Tool are not as efficient for this task.
Step-by-step explanation:
The correct tool to select the stars in a single step on a graphic design platform such as Adobe Illustrator and then change their fill color to a specific CMYK value is the Magic Wand Tool (option b). The Magic Wand Tool allows you to select objects with similar attributes such as fill color, stroke weight, opacity, etc., with just one click. After selecting the stars, you can change their fill color by updating the fill settings in the color panel to C=100, M=80, Y=25. This cannot be achieved with the Direct Selection Tool, Shape Builder Tool, or Eyedropper Tool as efficiently for this specific task.