The correct answer is A) clamp-on ammeter. A clamp-on ammeter is a useful instrument for checking 3-phase AC motor performance by measuring possible unbalanced currents. It can easily measure the current in each phase of the motor without having to disconnect the wiring, allowing you to quickly identify any significant imbalance between the phases. This is important because unbalanced currents can indicate issues with the motor or its power supply that could lead to reduced efficiency, overheating, and potential failure. The other options are not as well-suited for this task. A vibrating-reed frequency meter (B) is used to measure frequency, not current. A D'Arsonval iron-vane probe (C) is a type of voltmeter, not an ammeter. And a megger (D) is used to test insulation resistance, not motor current balance.