The correct answer is option D.The coal supply lasts the longest when there is no population growth.
As we know that coal reserves are non renewable resources. The formation of coal reserves took million of years. So once they are used up they will never ever be available again. The more is the population, the more will be the requirement of energy resources like coal reserves to sustain such large population. So, high population will tend to use more and more coal to meet daily energy requirement. But if there is no population growth, that time only existing population which is fixed & comparatively less will use the coal resources and therefore, the coal supply will last for a longer time.
