Question

which temperature resistance of metallic conductor becomes zero?

Answer 1

Resistance increases, in case of conductor, with increase in temperature and decreases, in case of insulator, due to increase in temperature. 

In case of conductor, the valance band and conduction band overlap with each other. So, there are excess electrons in the conduction band of a conductor. When you increase the temperature, more electrons will go to the conduction band from valance band by absorbing the energy. So, the conduction band becomes crowded and there will be more collision between the electrons which increases the resistance. So, with increase in temperature, the resistance of conductor increases. 

But in case of insulator, there is a large energy gap between the two bands. So, if the temperature rise is high, the electrons will go to the upper band. But since it is not crowded due to less number of electrons, so the conductance will increase due to availability of free electrons in the conduction band. So, the resistance decreases with increase in temperature in insulator.