The idea that regions are fluid means that

The idea that regions are fluid means that they are not fixed or static, but rather constantly changing and influenced by various factors. This can include physical factors such as climate and geology, as well as human factors such as culture and politics. Option A is incorrect as it refers to the theory of plate tectonics, which is a separate concept. Option B is also incorrect as it refers to the importance of freshwater for life, but does not relate to the fluidity of regions. Option C is the correct answer as it accurately describes the concept of fluid regions being influenced by people's perceptions and perspectives. Therefore, the correct answer is C.