The instrument used for measuring electric current is:
A. Ammeter
B. Galvanometer
C. Voltmeter
D. Potentiometer

Hint: Unit of Electric current is amperes and the instrument is named similar to the unit of electric current. Other options are instruments to measure either voltages or indicator of presence of current in the circuit.

Complete answer:
An instrument used to measure electric current is called an ammeter. The instrument displays the current passing through it in amperes and thus the name is ammeter. Generally the resistance of the instrument is very small and thus there is very less voltage drop across the instrument. This instrument is connected in series to other instruments across which we want to measure the current.
If we see other options then option C and D are instruments to measure voltage units. Galvanometer helps us to detect the presence of any electric current in the circuit. It is many times used to measure zero current in bridges. Thus option A is the correct one.

Note: Generally the names of the instruments really go together with the purpose it serves which in above case is amperes, unit for electric current. Now Potentiometer and voltmeter are used to measure voltages in one way but the first one is used to measure EMF in the circuit while the later one voltage drops at two points.