(a) Helium, Neon, and Argon are the examples of the special group of elements called the noble gases.
(b) This is because of their completely filled valence shell. All these elements have a valency of zero because of their fully filled outermost shells.
(c) Their atomic numbers are 2, 8 and 18 respectively, by their electronic configuration Helium has 2 electrons in the K shell, Neon has 8 in the L shell and Argon has 8 in the M shell.
(d) The K shell in the Helium is filled and the L and M shells in Neon and Argon respectively have a complete octet of electrons which makes them stable, so they do not require sharing their electrons and have the valency of zero.