**Core Concept**
The stability of an instrument during scaling and root planning is crucial for effective removal of plaque and calculus from the root surface. The stability of an instrument depends on its design, material, and the forces applied during the procedure. In the context of scaling and root planning, the stability of an instrument is influenced by its ability to maintain contact with the root surface and resist lateral forces.

**Why the Correct Answer is Right**
The ultrasonic scaler is designed to produce high-frequency vibrations, which allow for effective removal of plaque and calculus from the root surface. The vibrations generated by the ultrasonic scaler also help to disrupt the biofilm and reduce the risk of damage to the root surface. The ultrasonic scaler's stability is highest during scaling and root planning due to its ability to maintain contact with the root surface and resist lateral forces. The high-frequency vibrations produced by the ultrasonic scaler also help to reduce the risk of instrument breakage and improve the overall effectiveness of the procedure.

**Why Each Wrong Option is Incorrect**
**Option A:** The hand scaler is a manual instrument that relies on the operator's skill and dexterity to remove plaque and calculus from the root surface. While the hand scaler can be effective, its stability is lower than that of the ultrasonic scaler due to the risk of instrument breakage and the difficulty in maintaining consistent contact with the root surface.
**Option B:** The sonic scaler produces lower-frequency vibrations than the ultrasonic scaler and is less effective at disrupting the biofilm. The sonic scaler's stability is lower than that of the ultrasonic scaler due to its lower frequency and reduced ability to resist lateral forces.
**Option C:** The air-polisher is a high-speed instrument that uses compressed air to remove plaque and calculus from the root surface. While the air-polisher can be effective, its stability is lower than that of the ultrasonic scaler due to the risk of damage to the root surface and the difficulty in maintaining consistent contact.

**Clinical Pearl / High-Yield Fact**
The ultrasonic scaler is a high-frequency instrument that produces vibrations that are effective at disrupting the biofilm and removing plaque and calculus from the root surface. The ultrasonic scaler's stability is highest during scaling and root planning due to its ability to maintain contact with the root surface and resist lateral forces.

**Correct Answer:** A. Ultrasonic scaler