Which test is used to assess a person's ability to follow a line with their eyes?

The Horizontal Gaze Nystagmus test is specifically designed to evaluate a person's ability to follow a moving object with their eyes. This test involves the subject tracking a stimulus, typically a pen or finger, as it moves side to side in front of their eyes. During this assessment, the examiner looks for signs of nystagmus, which is an involuntary eye movement that can indicate impairment, often related to alcohol or drug use.

This test is particularly effective because it directly examines the eyes' tracking ability, a critical function for assessing coordination and potential intoxication. The other tests listed, while assessing various aspects of balance and coordination, do not focus specifically on the ocular tracking function like the Horizontal Gaze Nystagmus test does. For instance, the Nine Step Walk and Turn, One Legged Stand, and Field Coordination Test focus on different areas of physical coordination and balance rather than eye movement specifically. Therefore, the skill assessed by the Horizontal Gaze Nystagmus test makes it the correct choice for this question.