Step 1: Focusing on how humans hear sound.
The human ear can detect sounds ranging from extremely faint to extremely loud.
The weakest sound we can hear and the strongest sound we can tolerate differ by millions of times in intensity.
Step 2: Problem with a linear scale.
If sound intensity were measured on a linear scale, the numerical values would become very large and inconvenient to use.
Also, the human ear does not respond linearly to sound intensity—it responds logarithmically.
Step 3: Logarithmic representation.
To match human hearing and simplify measurements, sound intensity is expressed using a logarithmic scale:
\[ L = 10 \log_{10} \left( \frac{I}{I_0} \right) \]
This scale compares any sound intensity \( I \) with a standard reference intensity \( I_0 \).
Step 4: Unit associated with logarithmic sound scale.
A logarithmic comparison of sound intensities is expressed using a special unit called the decibel (dB).
This unit is universally used in acoustics, noise control, and environmental sound measurements.
Step 5: Eliminating other units.
(A) Hertz → Measures frequency, not loudness.
(C) Pascal → Measures pressure, not intensity level.
(D) Joule → Measures energy, not sound perception.
Only decibel correctly represents sound intensity level.
Final Conclusion:
The unit used to measure sound intensity level is:
\[ \boxed{\text{Decibel (dB)}} \]