Classification — music terms vs unrelated word: choose the single item that does NOT belong with the musical terms.

Introduction / Context:
Three options are standard music-related terms; one is an everyday adjective. Your task is to spot the outsider.

Given Data / Assumptions:

• Nocturne → a musical composition inspired by or evocative of night.
• Opus → work; in music, a cataloging/numbering term for compositions.
• Composition → a musical piece.
• Nocturnal → adjective meaning “active at night”; not a musical noun/term.

Concept / Approach:
Check part-of-speech and domain: three are nouns used in musical contexts; one is a general English adjective unrelated to music taxonomy.

Step-by-Step Solution:
1) Identify musical nouns: Nocturne, Opus, Composition.2) Examine “Nocturnal”: not a music label; it is an adjective about time of activity.3) Therefore “Nocturnal” is the odd word.

Verification / Alternative check:
Prepend “piano ___”: piano nocturne/opus/composition make sense; “piano nocturnal” does not as a genre label.

Why Other Options Are Wrong:
They are all entrenched in music theory/cataloging and thus form the majority group.

Common Pitfalls:
Confusing nocturne with nocturnal due to shared root “noct-”. Only “nocturne” is the music form.

Final Answer:
Nocturnal