Final answer:
The vertical seated position in which the dental chair's back is upright at a 90-degree angle is termed the upright or 90-degree position, promoting good posture and preventing back strain.
Step-by-step explanation:
The vertical, seated position where the back of the dental chair is upright at a 90-degree angle is often referred to as the upright position or the 90-degree position in the dental context. In this position, a patient's back is aligned perpendicularly to the seat of the chair, promoting good posture.
This posture aligns the patient’s center of gravity above the hips, helping to avoid back strain and potential damage to the spinal discs during dental procedures. This position is contrasted with other dental chair positions, such as the supine or reclined position, where the chair is laid back, and the patient's back is horizontal or nearly horizontal to the ground. Neither the story excerpt (b) about a person standing nor (c), which describes the supine position with upper and lower limbs touching the sides, corresponds to the vertical seated position at a 90-degree angle.