Final answer:
The haunted attraction likely improved participants' moods by providing a thrilling context in which unanticipated physiological arousal was interpreted as positive emotions, fostering a sense of euphoria and enjoyment.
Step-by-step explanation:
The detail that best illustrates why the haunted attraction improved participants' moods can be drawn from the principle of emotional arousal and contextual interpretation of emotions. The haunted attraction, with its myriad of sensorial stimuli, may act as an external context that people interpret in line with the arousal they experience. This interpretation, based on Schachter & Singer's research, suggests that when individuals are not anticipating the physiological arousal, they are more likely to contextualize it as an emotional change based on the surroundings. Therefore, the thrill and excitement induced by the haunted attraction could be perceived as euphoric or pleasurable emotions, enhancing mood.