The correct answer is - More people die than are born during a given period.
The negative population growth can be described as a situation in which more people die, or are emigrating, than there is people that are born, or immigrating in certain area/country in a precisely determined period.
This kind of trend in the demographics of a population leads to aging population, and than to declining population. This is often the case in the developed countries, where there's less and less births, there's more and more middle aged and elderly people, thus the population is aging. Eventually, the elderly people will start to die, so there will be a very quick decline in the population.
This is a trademark scenario of the fourth demographic transition model.