Final answer:
The masquers were 'happy and dauntless and sagacious' before the apparition of the Red Death, indicating a blissful and confident ignorance of the dangers they would soon face.
Step-by-step explanation:
The phrase that best expresses the mental state of the masquers before the apparition of the Red Death is "happy and dauntless and sagacious". This description indicates that the masquers were in a state of blissful ignorance and self-confidence, unaware of the impending danger posed by the Red Death. It contrasts sharply with the other phrases, which describe reactions to fear, horror, and despair, which are not the emotions they were experiencing before the Red Death's arrival.