Which of these tests requires the subject to balance on one leg?

The One Leg Stand test specifically requires the subject to balance on one leg. This test is commonly used in field sobriety assessments to evaluate a person's balance, coordination, and ability to follow instructions. During the test, the individual is instructed to stand on one leg for a specified duration, typically while counting. This task challenges both physical balance and cognitive function, as the subject must focus on maintaining stability while also keeping track of the count.

In contrast, the Walk and Turn test involves walking in a straight line, turning around, and returning in a specific manner, but does not require balancing on one leg. The Horizontal Gaze Nystagmus test assesses eye movements and is focused on detecting eye jerks as a person follows a moving object with their gaze, rather than physical balance. A Breathalyzer test measures blood alcohol content through breath analysis and does not involve any physical balancing tasks. Therefore, the One Leg Stand is the only option requiring the specific action of balancing on one leg.