Correct Answer (Detailed Explanation is Below)
B.
Infrasonic sound
Detailed Explanation
Infrasonic sound has a frequency below approximately 20 Hz. Humans generally cannot hear such sounds, although some animals can detect low-frequency sounds. Natural sources include earthquakes, volcanic activity and some atmospheric phenomena. Remember the distinction:
infrasonic < 20 Hz, audible ≈ 20 Hz–20 kHz, ultrasonic > 20 kHz.