The correct answer is (a). A phonometer is a device used to measure the intensity of sound. It is also known as a sound level meter or a decibel meter. Phonometers are used in a variety of applications, including noise control, audio engineering, and environmental monitoring.
(b) is incorrect because a phonometer cannot measure extremely high temperatures. A pyrometer is a device used to measure high temperatures.
(c) is incorrect because a phonometer cannot measure the frequency of electromagnetic waves. A frequency meter is a device used to measure the frequency of electromagnetic waves.
(d) is incorrect because a phonometer cannot measure atmospheric humidity. A hygrometer is a device used to measure atmospheric humidity.