The correct answer is: C. Their electronic configuration is stable.
Rare gases are generally chemically inert because they have a full valence shell of electrons. This means that they do not need to gain or lose electrons in order to achieve a stable electron configuration. As a result, they are not very reactive and do not form many chemical bonds.
Option A is incorrect because rare gases are not always monoatomic. For example, xenon can form a few stable compounds, such as xenon tetrafluoride (XeF4).
Option B is incorrect because rare gases do not always have low ionization capacity. For example, helium has a very high ionization energy, which means that it is very difficult to remove an electron from its atom.
Option D is incorrect because rare gases do not always contain electronic affinity. Electronic affinity is the energy released when an electron is added to a neutral atom. Rare gases have very low electronic affinities, which means that they do not readily accept electrons.