An earthquake produces infrasound before the main shock wave begins.
Explanation: Infrasound refers to low-frequency sound waves that are below the audible range for humans, typically with frequencies less than 20 Hz. Earthquakes generate these low-frequency vibrations as the tectonic plates move, causing the ground to shake.
Why other options are incorrect:
(a) ultrasound:
Ultrasound has frequencies higher than the audible range, meaning humans cannot hear them. While certain animals like bats can detect ultrasound, they are not relevant to the context of earthquakes.
(c) audible sound:
Audible sound is within the frequency range that humans can hear (approximately 20 Hz to 20 kHz). Earthquakes primarily produce sounds much lower than this.
(d) none of the above:
This is incorrect, as infrasound is a type of sound produced by earthquakes.