The true statement among the options provided is: Their atomic masses differ.
Neon has several isotopes, specifically neon-20, neon-21, and neon-22, which differ in the number of neutrons they contain. However, they all have the same number of protons (10 protons), which is characteristic of the element neon. Since they are stable isotopes, they are not radioactive. Therefore, the most accurate statement is that their atomic masses differ due to the different numbers of neutrons.