Soils vary throughout the world because

a. the composition is based on the parent rock in that location
b. people use soil differently
c. animals use soil in different ways
d. some countries do not have legislation protecting soil

A. the composition is based on the parent rock in that location.

Soil is formed by the process of withering of rock. Withering is a process of break down of rock into small fragments and further into small particles which aggregate in the form of soil by the action of agents like water and wind. The type of rock  in a particular region may not be found other regions of the world. Also, the soil composition varies depending upon the type of parent rock withered to form that soil. Therefore, soil vary throughout the world because the composition is based on the parent rock in that location.