The correct answer is: A. secondary instrument.
An ammeter is a device used to measure electric current. It is a secondary instrument, which means that it measures the current indirectly by measuring the voltage drop across a known resistance.
A primary instrument is one that measures the quantity of interest directly, without the need for any intermediate measurements. For example, a voltmeter is a primary instrument that measures voltage directly.
A secondary instrument is one that measures the quantity of interest indirectly, by measuring some other quantity that is related to the quantity of interest. For example, an ammeter measures current indirectly by measuring the voltage drop across a known resistance.
A recording instrument is one that records the quantity of interest over time. For example, a chart recorder is a recording instrument that records voltage over time.
An integrating instrument is one that integrates the quantity of interest over time. For example, a watt-hour meter is an integrating instrument that integrates power over time.