Identify the device used to change the resistance in an electric circuit.

The correct answer is Rheostat.

To understand why a rheostat is the correct device, let us look at the functions of each device listed in the options:

1. Rheostat: A rheostat is an adjustable or variable resistor. It is specifically designed to change the resistance in an electric circuit, which in turn allows control of the flow of electric current through the circuit without interrupting it.

2. Ammeter: An ammeter is an instrument used to measure the electric current flowing through a circuit. It does not change the resistance of the circuit (in fact, an ideal ammeter has zero resistance to avoid affecting the circuit).

3. Conductor: A conductor is a material (like copper or aluminum wire) that allows electric current to flow through it easily. It is a material component, not a control device used to dynamically change resistance.

4. Voltmeter: A voltmeter is an instrument used to measure the electrical potential difference (voltage) between two points in a circuit. It has very high resistance to avoid drawing current, but it is not used to vary the resistance of the circuit.

Therefore, the device used specifically to change the resistance in an electric circuit is the Rheostat.