Introduction:
When the temperature of a conductor increases, its resistance also increases. This phenomenon is known as the temperature coefficient of resistance. The increase in resistance with temperature can be explained by the changes in the behavior of the conducting electrons within the material.

Explanation:
There are several factors that contribute to the increase in resistance with temperature, but the most significant one is the decrease in relaxation time.

Relaxation Time:
The relaxation time is the average time between two successive collisions of an electron with the atoms of the conductor. It determines how long an electron can travel before it collides with an atom and loses its momentum.

- When the temperature of a conductor increases, the atoms in the material vibrate more vigorously. These vibrations can disrupt the path of the electrons and increase their chances of colliding with the atoms.
- As a result, the average time between collisions decreases, leading to a decrease in the relaxation time.
- A shorter relaxation time means that the electrons have less time to travel through the material before they collide, resulting in an increase in resistance.

Other Factors:
While the decrease in relaxation time is the primary cause of the increase in resistance with temperature, there are other factors that can also contribute to this effect:

1. Mass of Electrons:
The mass of the electrons does not change with temperature. Therefore, it does not directly affect the resistance of the conductor. Hence, option 'B' is incorrect.

2. Electron Density:
The electron density refers to the number of free electrons per unit volume in the conductor. When the temperature increases, some of the electrons gain enough energy to break free from their atoms and become free electrons. This increase in electron density can lead to a decrease in resistance. Thus, option 'C' is also incorrect.

Conclusion:
In conclusion, the increase in the temperature of a conductor leads to an increase in resistance primarily due to the decrease in relaxation time. The increase in electron density and the mass of electrons do not directly affect the resistance. Therefore, option 'A' is the correct answer.