Correct Answer: cosmic radiation

Solution : The correct option is cosmic radiation.

Victor Franz Hess is best known for his pioneering work in the field of cosmic rays and, in particular, for his discovery of cosmic radiation. In 1912, Hess conducted a series of balloon flights and observed that the level of ionising radiation increased with altitude, which was contrary to the prevailing scientific belief at the time. This discovery ultimately led to the understanding that cosmic rays, high-energy particles from space, penetrate the Earth's atmosphere and contribute to the background radiation on Earth.