Which marking can extend down to the nostrils on a horse?

A bald face is a distinctive marking on a horse where the white markings extend over the bridge of the nose and may reach down to the nostrils. This marking creates a striking appearance, often giving the horse an almost 'face mask' effect. The extent of white can vary from horse to horse; however, the key characteristic is its coverage over a large portion of the face, including reaching the nostrils.

In contrast, the other options refer to different types of markings. A half pastern typically describes white marking that extends just above the fetlock; a sock details a white marking that reaches up to the ankle or pastern area; and a star denotes a small white mark on the forehead. None of these markings extend down to the nostrils, which highlights why the bald face is the correct identification for the situation described.