Final answer:
The word 'infect' best completes the sentence, indicating that Aardvark's negative mood spread to others, similar to the moody depression that changed the atmosphere in the provided context.
Step-by-step explanation:
The sentence in question should be completed with a word that indicates how the Aardvark's mood affected everyone else.
Based on the context provided, where a moody and depressive atmosphere is described, a suitable word is infect.
So the sentence would be: "Aardvark's grumpiness infected everyone else's mood last night.
" This suggests that the negativity spread from Aardvark to the others, just as a person's mood can have a contagious effect on those around them, similar to the scene described where a character's uneasiness and disturbance due to another's unpredictable moods changed the overall atmosphere.