Which of the following tools is important for smoothing hair?

The flat iron is an essential tool for smoothing hair because it utilizes heat to straighten and tame frizz, creating a sleek and polished appearance. When the plates of the flat iron make contact with the hair, they effectively close the hair cuticles, resulting in a smooth, shiny finish. This process is particularly effective on various hair types, including those that are curly or wavy, allowing for transformative styling in a relatively short amount of time.

While round brushes and combs are useful for styling and managing hair texture during blow-drying or combing, they do not provide the same level of smoothing that a flat iron does. A round brush can create volume and shape but requires heat from a blow dryer to work effectively. A comb is primarily used for detangling and can help with some styling but lacks the ability to create a straightened effect. A diffuser, typically attached to a blow dryer, aids in enhancing curls and reducing frizz but does not directly smooth hair like a flat iron does. Therefore, for achieving a straight and smooth finish, the flat iron stands out as the most effective tool.