How the stability of the oxidation states of group IV element vary down the group. (Illustrate your answer with the chloride of carbon and lead) 

Oxidation state +2 become more stable down the group

Oxidation state +4 become less stable down the group.

This can be illustrated by chlorides of carbon and lead as follows:

CCl₄ is stable whereas PbCl₄  easily decomposes on heating giving lead (II) chloride and chlorine

PbCl₄ (g) (heat) →PbCl₂ (s) + Cl₂ (g)