Which type of brush is primarily designed for adding volume during blow drying?

Round brushes are specifically designed to create volume and enhance the natural shape of the hair during blow drying. Their cylindrical shape allows for effective curling and lifting of the hair strands at the roots as heat is applied. The round design enables stylists to wrap sections of hair around the brush, which helps in forming smooth waves or curls while adding bounce and volume.

In contrast, vented brushes have openings that promote airflow, making them ideal for drying but not specifically tailored for volumizing. Flat brushes are more suited for straightening and smoothing hair rather than creating volume, as they lay the hair flat rather than lifting it. Grooming brushes are primarily used for finishing and smoothing hair, rather than for styling during the drying process. Thus, the effectiveness of round brushes in creating volume during blow drying makes them the correct choice.