Final answer:
The line indicating that characters moved to the Moon and celebrated anniversaries there is a clear indicator of science fiction.
Step-by-step explanation:
The line that provides the strongest evidence that the selection is a work of science fiction is: "They had come there for every anniversary after they moved to the Moon—36 anniversaries in all." This line immediately suggests a setting and scenario that are not possible based on our current reality, thereby classifying the narrative in the genre of science fiction, where such imaginative and futuristic concepts are a staple.