It is the amount of sound energy passing each second through a unit area.
Explanation
Sound waves transport energy through a medium. The magnitude of this energy flow is physically quantified as "Intensity." It is an objective property of the wave, distinct from subjective perception.
Detailed Analysis:
- It is the physiological response of the ear to sound. is incorrect: The physiological response of the ear to sound is termed Loudness. Loudness is subjective because it depends not only on the sound's intensity but also on the sensitivity of the human ear to specific frequencies.
- It is the amount of sound energy passing each second through a unit area. is correct: Intensity is defined as the amount of sound energy passing each second through a unit area held perpendicular to the direction of propagation. Since energy per unit time is Power, intensity is also defined as Sound Power per unit area ($I = \frac{P}{A}$). The SI unit is watt per square meter ($W/m^2$).
- It is the total distance travelled by a sound wave in one second. is incorrect: The distance travelled by a wave in one second is the Speed (or velocity) of sound.
- It is the number of oscillations per unit time. is incorrect: The number of oscillations or vibrations per unit time is the Frequency of the sound, measured in Hertz (Hz).
Key Takeaway:
Intensity is an objective, measurable physical quantity ($W/m^2$) representing energy flow, whereas Loudness is the subjective perception of that intensity by the human ear.