The answer is: B. ionizing radiation is present.
A Geiger counter is an instrument used for detecting and measuring ionizing radiation.
The Geiger counter is filled with an inert gas (noble gases) at low pressure, it conducts electrical charge when a particle or photon of radiation makes the gas conductive by ionization.
For example, it can detect alpha particles.
Alpha particle is nucleus of a helium-4 atom, which is made of two protons and two neutrons.