Which technique is often used to apply highlights?

The technique of foiling is commonly used to apply highlights because it allows for precise placement of the hair color. In this method, thin sections of hair are placed on a sheet of aluminum foil along with the highlighting product. This technique helps to isolate each highlight from the surrounding hair, ensuring clean and well-defined highlights. The foil also helps to create heat, which can accelerate the processing time of the color, resulting in a more vibrant finish.

Brushing involves applying color through brushing motions, which can be less precise and may not achieve the same level of definition as foiling. Blow-drying is primarily a styling technique used to dry hair and create a desired look, while washing is a cleansing process and does not pertain to the application of highlights at all. Therefore, foiling stands out as the most effective and commonly used method for achieving highlights, making it the correct answer.