Step 1: Recall the function of each device listed.
Wattmeter: Measures electric power (in Watts).
Voltmeter: Measures electric potential difference (voltage) between two points in a circuit (in Volts).
Ammeter: Measures electric current flowing through a point in a circuit (in Amperes).
Resistor: An electrical component that opposes the flow of electric current.
Step 2: Identify the device used to measure electric current.
Based on the definitions above, the ammeter is the device specifically designed to measure electric current.