Concept:A gas conducts electricity when free electrons and ions are available to carry charge.
Explanation:At normal pressure, gas molecules collide too often, so free charges cannot move easily and the gas behaves as an insulator.
When the pressure is low, the gas becomes less dense, so electrons travel longer distances without hitting molecules.
A high voltage gives these electrons enough energy to ionize neutral gas molecules by knocking off more electrons.
This ionization produces positive ions and additional free electrons, which create a conductive path.
Thus, the best condition for electrical conduction through a gas is low pressure with high voltage.
Answer:D. Low pressure & high voltage.