The correct choice is (c) carrier amplifiers
For explanation I would say: Carrier amplifiers are used with transducers which require an external source of excitation. They essentially contain a carrier oscillator, a bridge balance and calibration circuit, a high gain ac amplifier, a phase-sensitive detector and a dc output amplifier. They are characterized by high gain, negligible drift, extremely low noise and the ability to operate with resistive, inductive or capacitive type transducers.