The correct answer is: B. Increases
The resistance of a tungsten lamp increases as the applied voltage increases. This is because the resistance of a conductor is proportional to its length and inversely proportional to its cross-sectional area. When the voltage is increased, the current through the lamp increases. This causes the filament to heat up and expand. The expansion of the filament increases its length and decreases its cross-sectional area, which in turn increases its resistance.
Option A is incorrect because the resistance of a conductor increases as its temperature increases. Option C is incorrect because the resistance of a conductor is not constant. Option D is incorrect because the resistance of a conductor does not decrease as the applied voltage increases.