What tool is commonly used for blow-drying hair?

A hairdryer with a concentrator nozzle is commonly used for blow-drying hair because it is specifically designed to direct and focus the airflow. The concentrator nozzle helps create a more concentrated stream of hot air, which allows for better control during the styling process. This feature is particularly useful for smoothing the hair, achieving specific styles, or adding volume, as it can be directed at particular sections of hair.

While round brushes are often used in conjunction with a hairdryer to style hair and create volume or curls, they do not emit heat or airflow on their own, which is essential for the blow-drying process. A flat iron and hair straightener are primarily used for flattening or straightening hair rather than drying it. Therefore, the hairdryer with a concentrator nozzle stands out as the most appropriate tool for blow-drying hair effectively.