Which type of instrument removes residual deposits of calculus?

The correct answer is a periodontal file. A periodontal file is specifically designed to remove residual deposits of calculus that may remain after the bulk of calculus has been scaled away. Its unique design features several sharp edges that allow it to effectively smooth the root surface and remove stubborn deposits, making it particularly useful in periodontal therapy. The application of a periodontal file helps in preparing the tooth surface for further treatment or to promote healing in the periodontal tissues.

The other instruments listed have different primary functions. An ultrasonic scaler, for instance, utilizes high-frequency vibrations to help dislodge calculus but is typically used for the initial removal of larger deposits rather than finer finishing. A curette is useful for scaling and root planing but is more aimed at removing debris and smoothing surfaces rather than specifically targeting residual calculus. A push scaler is designed for scaling but does not offer the specialized capability to address remaining calculus as effectively as a periodontal file.