Which statement best captures the distinction between Sunnah and nafl here?

Classes of Normative Rulings: Fard, Wajib, Sunnah and Nafl

• Sunnah has no reward but nafl does
• Nafl is always binding
• Omission of nafl always earns blame
• Emphasized Sunnah may require maintained observance; pure nafl omission is not punishable