Answer: The word "quixotic" is most clearly defined as "impractical".
Step-by-step explanation: The word "quixotic" is an adjective and it is mainly used to describe someone as unnecessarily idealistic. In other words, "quixotic", which derives from the character "Don Quixote", can be defined as "impractical and unrealistic". Therefore, the adjective "rational" cannot be applied to this word. Moreover, "sentimental" and "insane" do not capture the real meaning of "quixotic" either.