The correct answer is A. 5 Hz.
Infrasound is sound with a frequency below the lower limit of human hearing, which is 20 Hz. It is produced by many natural and artificial sources, including earthquakes, volcanoes, thunder, wind, ocean waves, and many animals, including elephants, whales, and bats. Infrasound can travel long distances through the air and water, and it can cause physical effects in humans, such as changes in blood pressure and heart rate.
Option B, 15 Hz, is incorrect because it is above the lower limit of human hearing. Option C, 20 Hz, is incorrect because it is the lower limit of human hearing. Option D, 32 Hz, is incorrect because it is above the lower limit of human hearing.