A high-gain, direct-coupled differential amplifier with very high input impedance and very low output impedance✓Correct answer
A digital audio amplifier whose characteristics are determined by components external to the amplifier
An amplifier used to increase the average output of frequency modulated amateur signals to the legal limit
A RF amplifier used in the UHF and microwave regions
Explanation
An operational amplifier is a direct-coupled differential amplifier with very high gain, very high input impedance, and very low output impedance. Those properties are what allow external feedback components alone to determine the circuit's behavior.