Which one of the following is the unit for measuring the intensity of sound?

The physical intensity of a sound wave — the rate of sound energy flowing through a unit area — is measured in W/m2. But this raw scale spans an enormous range and does not match how the human ear perceives loudness, so sound intensity is conventionally expressed on a logarithmic scale relative to a reference threshold (about 10−12 W/m2, the threshold of human hearing), using the decibel (dB), a unit accepted for use with the SI. This logarithmic quantity is called the sound intensity level.

Applying this to the question: among the four options, decibel is the unit conventionally used whenever a sound's intensity (loudness) is quoted — this is the standard convention followed in exams and general usage, and is confirmed by the official answer key for this paper, even though the raw physical intensity itself would strictly be stated in W/m2, a unit not offered among the choices.

Cross-checking against the other options confirms this by elimination, since each measures an entirely different physical quantity:

• Coulomb is the SI unit of electric charge (the charge carried by a current of one ampere flowing for one second) — it has no relation to sound.

• Henry is the SI unit of electrical inductance (the property of a circuit that opposes a change in current) — also unrelated to sound.

• Hertz is the SI unit of frequency (cycles per second) — it measures the pitch of a sound, not its intensity or loudness.

So the unit conventionally used for measuring the intensity of sound is the decibel.