What is a desert?
a. a flat area covered primarily with sand
b. an area of extreme heat year round
c. an area that receives very little precipitation
d. an area where there is rain during only one month of the year

A desert is an area that receives very little precipitation - c.

Deserts are mostly areas with sand, as is popularly considered, but it's not only sand that is problematic, it is also the fact that these areas receive very little precipitation (rain) which poses more of a problem.