Concept:Valency is the combining capacity of an atom; noble gases have a completely filled outer shell and hence zero valency.
Explanation:Atoms combine to form compounds by sharing or transferring valence electrons.
The number of electrons in the outermost shell determines the valency.
Noble gases like helium, neon, argon, and radon have a full outer electron shell.
This full shell makes them chemically inert, so they do not form bonds.
Therefore, all noble gases, including radon, exhibit zero valency.
Answer:D. Radon