Final answer:
A toner is a hair coloring product that is used after the initial hair color formula to enhance or adjust the color of the hair.
Step-by-step explanation:
The correct answer is d) Toner. A toner is a hair coloring product that is used after the initial hair color formula to enhance or adjust the color of the hair. In this case, the prepared tint and shampoo mixture is added to the remaining color formula to brighten the hair color before rinsing. It is like a final touch to achieve the desired shade of hair color.