1. The problem statement, all variables and given/known data 2. Relevant equations Between Radio Waves and Protons is where my confusion lies; Online I read that Radio waves are non ionizing but if that's the answer then how are protons ionizing? I mean it's no even included in the types of ionizing radiation here: https://en.wikipedia.org/wiki/Ionizing_radiation#Types 3. The attempt at a solution X-rays are among the high energy electromagnetic waves so are ionizing, B is wrong. Alpha particles are charged and heavy so are ionizing, C is wrong.