The correct answer is option 4 - Radon. Valency refers to the combining capacity of an element, or the number of electrons an atom needs to gain, lose, or share in order to achieve a stable configuration. It determines how an element will participate in chemical reactions and form compounds.

Radon is a noble gas in the periodic table, located in Group 18. Noble gases are known for having a full outermost electron shell, which makes them generally unreactive and stable. They already have a complete octet, so they do not need to gain, lose, or share any electrons.

On the other hand, options 1, 2, and 3 (Sulphur, Phosphorous, and Lead) are not noble gases. They belong to different groups and have valencies other than zero. Sulphur and Phosphorous can both have various valencies depending on the chemical compound they are present in, while Lead usually has a valency of +2 or +4.

In summary, Radon is the element with zero valency because it is a noble gas with a full outer shell of electrons.