Answer: C. Magnets cause the ions to conduct electricity.
Explanation:
A Geiger counter is used to detect radioactive decay such as gamma rays, beta particles. It is made up of a tube which consists of gas. The ends of the tube are anode and cathode. The radioactive decay ionizes the gas. Ionized gas produces an electric current which is detected and measured.
Thus, C. Magnets cause the ions to conduct electricity is not a step in operation of a Geiger counter.