Answer:
D. "certain low and indefinite sounds"
Explanation:
In the passage, the detail that changes the mood from unease to horror is the "certain low and indefinite sounds".
When the low and indefinite sounds are heard the mood of even the readers actually change to horror. Those type of low and indefinite sounds are actually what makes movies, poems or stories have the horror genre and become suspense-filled.
Though this question didn't carry enough information about the passage. I answered this with the best of my knowledge.
