why do elements like copper iron show variable valency .plzzz explain.............

Because copper and iron have same number of electrons in outermost orbit

The IUPAC definition limits valence to the maximum number of univalent atoms that may combine with the atom, that is the maximum number of valence bonds that is possible for the given element.
The oxidation state of an atom in a molecule gives the number of valence electrons it has gained or lost. In contrast to the valency number, the oxidation state can be positive (for an electropositive atom) or negative (for an electronegative atom). 
Not only d block elements have variable valency, Oxygen, Nitrogen, etc., some inert pair effected elements also having variable oxidation state

i think this is due to inert pair effect because when there is an increase in numbr of orbitals the electrons of outer most shell do held tightly and hence they do show variable oxidation states. infact not only copper, iron, nd d- blok elements but others do so for example thallium, lead, etc. summing up the d nd f orbitals show poor screening effect. hence the case so.

because they have variable electronic configuration