The science that describes populations is called gerontology psychology demography geography

We can identify which of the scientific studies describe population thru their definitions.

Gerontology is particular with old age, its process and the problems are encountered by old people.

Psychology studies the feeling, behavior, and mind of people, including all phenomena.

Demography shows the structure of human population, how it changes, based on the statistics of births, deaths, income, or occurrence of disease, the composition of a particular human population.

Geography is understanding of the Earth.  And it's complexities.

Correct answer then is demography.

answer; ///i believe the correct answer is (aids) acquired immune deficiency syndrome;

the form of energy changes and the total energy increases0l