Electrical Instruments

The resistance of a conductor increases with temperature primarily because of the increased collision frequency between the free electrons and the atoms of the conductor. As the temperature increases, the atoms vibrate more, which leads to more frequent collisions with electrons, reducing the relaxation time, which is the time between these collisions. The increased collision rate results in a higher resistance.
Therefore, the correct reason for the increase in resistance is that the relaxation time decreases as temperature increases.