I’d say A: Dave is partly right in his answer that gamma rays are the most dangerous, but it is because of their frequency and wavelength, not where they come from.
Gamma rays are the most intense and thus, most harmful electromagnetic waves. Gamma rays radiation poisoning is difficult to shield against. These rays have the most energy and can go through six feet of concrete and damage your DNA as well. The higher the energy waves (gamma, x-ray), the shorter the wavelengths