Mr sanchez owns a square field. the side lengths are 0.9 kilometers. there are 1980 prairie dog burrows in the field. what is the approximate density of prairie dog burrows in burrows in square kilometers

Density is the measurement of the amount of mass per unit of volume. 
In this case we should calculate the density of prairie dog burrows on a square field. This is a surface charge density.It is defined as the total amount of units q per km^2.
to calculate the density of prairie dog burrows, we should divide the total number of  prairie dog burrows in the field by the size of the field. Thus,

 prairie dog burrows in square kilometers= number of dog burrows/ size of the field

size of the field=0.9*0.9=900*900=180000m^2
1980/180000 m^2= 0.011 dog burrows in square meter 
11 dogs in square kilometer

The answer would be choice b