You are correct, the answer is C) current transformer. The clamp-on AC ammeter uses a current transformer to measure the current flowing through a conductor. The split-core design allows the ammeter to be clamped around the conductor without interrupting the circuit. The current transformer steps down the high current to a lower value that can be measured by the rectifier-type instrument connected to the secondary winding. The other options are incorrect because a control transformer is used for voltage regulation, a reactance transformer is used for power factor correction, and a potential transformer is used to measure voltage, not current.