Why do deserts make movement difficult? which of the five themes applies to this situation?

1. What make a moving object to stop is the amount of friction that stop the force from moving into a certain way.
The sands particle that exist on the desert provide a lot of friction for moving objects, which make the movement on top of desert really difficult whether you're on foot or using a vehicle.

2. They provide water and transportation for the region. This applies to the theme place. Since it is very difficult to move on top of  a land in a desert region, water transportation become widely used.
Because of this, rivers such as  Tigris and Euphrates rivers become really important in such region.

Ithink b is the right answer

Dcalifornia they have the most people with 200+ people per square mile