The correct answer is: C. Intensity of sound.
A decibel (dB) is a logarithmic unit that expresses the ratio of a physical quantity (such as sound pressure, power, or voltage) to a reference value. The reference value is often 1 milliwatt (mW) for power, 20 micropascals (μPa) for sound pressure, and 1 volt (V) for voltage.
Decibels are used to express a wide range of quantities, from the intensity of sound to the power of radio waves. They are also used to express the gain or loss of a signal in a communication system.
The decibel is a convenient unit because it is a dimensionless quantity. This means that it can be used to express the ratio of any two quantities, regardless of their units.
The decibel is also a relative unit. This means that it expresses the ratio of a quantity to a reference value, rather than the absolute value of the quantity itself. This makes it useful for comparing quantities that have different units or that are measured on different scales.
The decibel is a powerful unit that can be used to express a wide range of quantities. It is a convenient and relative unit that is used in a variety of fields.
Option A is incorrect because the speed of light is measured in meters per second (m/s).
Option B is incorrect because the intensity of heat is measured in watts per square meter (W/m2).
Option D is incorrect because the frequency of radio waves is measured in hertz (Hz).