Correct answer is (d) audiometer
The best I can explain: An audiometer is specialized equipment, which is used for the identification of hearing the loss in individuals, and the quantitative determination of the degree and nature of such a loss. Both frequency and output are adjustable over the audio range. The instrument is also provided with a calibrated noise source and bone-conductor vibrator. It is essentially an oscillator driving a pair of headphones and is calibrated in terms of frequency and acoustic output.