You are correct, the answer is D) current transformer. The clamp-on AC ammeter uses a current transformer to measure the current flowing through a conductor. The current transformer has a split-core design that allows it to be clamped around the conductor without interrupting the circuit. The current transformer steps down the high current in the conductor to a lower current that can be safely measured by the rectifier-type instrument connected to the secondary winding. The other options are incorrect because they do not match the functionality of a current transformer. A control transformer is used for voltage regulation, a reactance transformer is used for power factor correction, and a potential transformer is used to step down high voltages for measurement purposes.