Final answer:
The audio aura of a listener's top 100 songs from 2018 would be characterized as eclectic if it includes a variety of musical genres and moods, since eclectic denotes a diverse and varied taste in music.
Step-by-step explanation:
Characterizing an audio aura based on the top 100 songs of a listener from 2018 would involve analyzing the emotional or energetic themes prevalent within the music selection. If the compilation includes a variety of genres, tempos, and artists, and doesn't lean towards a specific mood consistently, the audio aura might be described as eclectic. An eclectic audio aura suggests a diverse and varied musical taste, embracing a wide range of styles and expressions. The other options listed such as energetic, melancholic, and nostalgic would require a more uniform collection of songs reflecting those moods—upbeat and fast-tempo for energetic, somber and slow for melancholic, or songs from past years indicative of a longing for the past for nostalgic.