Answer:
thermal conductivity
Step-by-step explanation:
An iron works by utilizing the heat generated when passing through a wire. Electricity flows due to the flow of electrons in a material. When the electrons collide with the atoms of the wire there is a transfer of kinetic energy which is realised as heat. If there are many collsions this means that the material has many collions then the material is a bad conductor. If there are fewer collisions then it is a good conductor.
In an iron we need this heat to be transferred to the metal plate of the iron. As metal is a good conductor of heat it is used in an iron.