Final answer:
Placing the target below the threshold on an EMG biofeedback unit is used to achieve muscle relaxation. Hence, option D is correct.
Step-by-step explanation:
Placing the target below the threshold on an EMG biofeedback unit is used to achieve muscle relaxation. Biofeedback is a technique that uses electronic equipment to measure a person's neuromuscular autonomic activity and provide feedback in the form of visual or auditory signals. By placing the target below the threshold, it encourages the individual to consciously relax their muscles.