The unit used for measuring noise pollution is:

Concept

Physical quantities are each measured in their own SI unit: force in newtons, energy/work in joules. Sound is different — its level is expressed on a logarithmic scale using the decibel (dB), a unit accepted for use with the SI to express sound-pressure level: twenty times the base-10 logarithm of a sound wave’s pressure ratio to a fixed reference threshold.

Application

"Noise pollution" refers to unwanted or excessive sound in the environment, so the quantity being measured here is sound level/intensity. Among the given options, the decibel (dB) is the unit built specifically to express that quantity, which is why it is the standard unit for measuring noise pollution levels (e.g. under India's Noise Pollution (Regulation and Control) Rules, 2000, and WHO environmental-noise guidance, limits are stated in dB(A)).

Cross-check / Contrast

• Joule measures energy or work done (e.g., mechanical or electrical work) — it has no relation to the pressure level of a sound wave.

• Newton measures force (mass × acceleration) — it describes a push or pull, not sound-pressure level.

• Nano unit is not an actual measurement unit; "nano" is merely a metric prefix (10⁻⁹) that must attach to a real unit (like nanometre), so it cannot stand alone as a measure of anything, let alone noise.

Result: the decibel is the only option among those given that is a genuine unit of sound/noise measurement.