Noble gas has van der waals radius because they are chemically very less reactive. Also there outermost orbitals are fulfilled with 8e- which cause very high repulsion between two atoms so noble gas prefers vanderwaals radius as it is non bonded radius i.e. atoms just touch each other.

Van der Waals radius is the correct answer.

Explanation:
- Atomic radius is the size of an atom, which is typically measured as the distance between the nucleus and the outermost electron orbital. However, noble gases have completely filled electron shells and do not readily form chemical bonds, so their atomic radius is not a suitable measure for them.
- Ionic radius is the size of an ion, which is typically measured as the distance between the nucleus and the outermost electron orbital of an ion. Noble gases do not readily form ions, so their ionic radius is not applicable.
- Metallic radius is the size of a metal atom, which is typically measured as the distance between the nuclei of adjacent metal atoms in a solid metal lattice. Noble gases are not metals, so their metallic radius is not applicable.
- van der Waals radius is the size of an atom or molecule, which is typically measured as the distance between the nuclei of neighboring atoms or molecules in a non-bonded state. This measurement takes into account the attractive and repulsive forces between atoms or molecules. Since noble gases exist as individual atoms in their elemental form and do not readily form chemical bonds, the van der Waals radius is a suitable measure for them.

In summary, the van der Waals radius is the appropriate measure of size for noble gases because it accounts for the non-bonded interactions between individual atoms in their elemental form.