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

Im probably super late by now- but the answer should be C... :)

c. a region is difficult to define because it is influenced by people's perceptions and perspectives

Explanation:

A region can be said to be characterized by the physical and human aspects and may be defined as part of land or area that is based on the homogeneous or heterogeneous characteristics of the time and depends on how people tend to derive meaning out of it. The nodal region, Heterogeneous region and A functional region all are made for some purpose and have a definite political zone or boundary. Thus regions are flexible like the fluids and can be easily molded for distinctive purposes and types.