The instrument used to measure electric current is

ammeter
electrometer
galvanometer
spectrometer

The correct answer is A. ammeter.

An ammeter is an instrument used to measure electric current in a circuit. It is connected in series with the circuit, and the current is measured by the deflection of a needle or the movement of a pointer on a scale.

A galvanometer is a device that detects and measures electric current. It is a type of ammeter, but it is more sensitive and can be used to measure very small currents.

An electrometer is a device used to measure electric charge. It is not used to measure current.

A spectrometer is a device used to measure the wavelengths of electromagnetic radiation. It is not used to measure current.