I apologize for the confusion. Among the given answer choices, "mysterious" would be the most suitable option for describing Walter Mitty based on the given statement.
The phrase "Walter Mitty the Undefeated, inscrutable to the last" is a description of a person named Walter Mitty. It suggests that he is strong and unbeatable, and that he is also difficult to understand or figure out. The options provided are possible words that could describe Walter Mitty based on the given phrase: devilish, mysterious, notorious, and stubborn. However, without further context or information, it is difficult to determine the exact word that best fits the description. It would be helpful to have more information about Walter Mitty's actions, personality, or background in order to make a more accurate assessment.
