Choose the statement that best describes population density.
an increase in the number of people living in an area over time
a decrease in the number of people living in an area over time
the number of people who live in a square mile or kilometer
the number of live births per one thousand people in a year

The answer is: [C]:  

           "the number of people who live in a square mile or kilometer" . 


Let us examine the other answer choices:

Answer choice: [A]: "an increase in the number of people living in an area over time" — refers to: "population growth".

Answer choice: [B]: "a decrease in the number of people living in an area over time" — refers to: "population decline"; or, "depopulation".

Answer choice [D]: "the number of live births per one thousand people in a year" — refers to: "birth rate". (as opposed to: "the number of deaths per one thousand in people in one year" — which refers to the "death rate"; or, "fatality rate").

What exactly do you mean 

The answer is i believe low birth rights.hope my answer is the one you are looking for : )