Final answer:
The mesial view is the only view in which only one root of a mandibular first molar is visible.
Step-by-step explanation:
The correct answer is A) Mesial view. From the mesial view, only one root of a mandibular first molar is visible. The mesial view shows the front of the tooth, facing the midline of the dental arch. In this view, you can see the mesial root, which is the root closest to the midline of the mouth. The distal root, on the other hand, is not visible from this view.