The correct answer is - D. all of the above.
In essence, spatial terms in geography refers to the describing of things that are relating to areas.
So population, trade flows, cities and towns, can all be observed as things in an area. Population can be seen area by area, so that the density, ethnicity, education etc can be seen and compared between areas. The trade flows usually are divided into trade zones, or trade areas, and cities and towns all belong to a certain area.