Final answer:
The term "incredulous" is best defined as a state of disbelief or skepticism, which indicates that a person is doubtful of the truth or reliability of something.
Step-by-step explanation:
The best meaning of "incredulous" as it is used in the sentence on page 66 is disbelief or skepticism. This word describes a state where someone finds it hard to believe what they are seeing, hearing, or being told. Neither excitement, anger, sadness, nor the other emotional states listed, such as fearfulness, happiness, or indifference, capture the essence of incredulity. An incredulous person is not necessarily scared or happy; they are simply doubtful of the truth or reliability of something and are showing or feeling skepticism.