The idea that regions are fluid means that
a. earth’s surface is floating on a hot magma layer
b. a region cannot support life without a source of freshwater
c. a region is difficult to define because it is influenced by people's perceptions and perspectives
d. formal geographic and political boundaries define a region

The correct answer is (c.) a region is difficult to define because it is influenced by people's perceptions and perspectives.

The region is defined as fluid because of the difficulty in defining the region due to perceptions and perspectives of the people. Like the liquid changed it's shape depends on the container, the region changed depends on the people.