What is the correct instrument to measure the final power amplifier current?

An ammeter is an instrument specifically designed to measure electrical current, which is measured in Amperes (or amps). In the context of a final power amplifier, you would place an ammeter in series with the power supply line to determine exactly how much current the amplifier is drawing. Looking at the distractors, a voltmeter measures electrical potential (voltage), an ohmmeter measures resistance, and a wattmeter measures overall power output (watts). Because the question specifically asks for a measurement of "current," the ammeter is the only correct choice.