What does "rareify" not relate to?

The term "rareify" is derived from the verb "rarefy," which is used to describe the process of making something less dense or thick. It can also imply a refinement or a process of purifying, particularly in a metaphorical sense, such as refining ideas or concepts. Therefore, making something less thick and reducing thickness align directly with the meaning of "rareify."

Conversely, increasing density is the opposite of what rareifying entails. When something is rarefied, it becomes less dense, not more. This foundational understanding of the term clarifies why increasing density is the correct answer regarding what "rareify" does not relate to.