Given that the patient has a poorly shielded cardiac pacemaker, the most appropriate instrument to use for subgingival scaling would be D) Curette.
Curettes are hand instruments used for scaling and root planing in periodontal treatment. They are a common choice when ultrasonic scalers cannot be used due to contraindications or patient-specific circumstances, such as the presence of a cardiac pacemaker.
Using a curette allows for manual removal of subgingival calculus without the risk of interference with the pacemaker. It provides control and precision in removing the calculus deposits while minimizing potential risks associated with electronic devices.
It is important to consider the patient's medical condition and any contraindications when selecting dental instruments to ensure their safety and well-being during the treatment.