The correct answer is A. Decibel.
A decibel (dB) is a logarithmic unit that expresses the ratio of a physical quantity (such as sound pressure) to a reference value. The reference value for sound pressure is 20 micropascals (μPa), which is the threshold of human hearing.
The decibel scale is a non-linear scale, which means that a change of 10 dB represents a tenfold increase in the physical quantity. For example, a sound pressure level of 90 dB is 10 times louder than a sound pressure level of 80 dB.
The decibel scale is also a relative scale, which means that it does not have an absolute zero. This is because the threshold of human hearing varies from person to person.
The decibel scale is used to measure a wide variety of physical quantities, including sound pressure, sound intensity, light intensity, and electrical power.
Option B, Decimal, is not a unit of noise pollution.
Option C, Ppm, is a unit of parts per million. It is not a unit of noise pollution.
Option D, None of these, is not the correct answer. The correct answer is A. Decibel.