Concept:Voluntary fasting refers to optional fasts, while obligatory fasts are those required by Islamic law, such as makeup fasts.Explanation:Sittah Shawwal, Ashura, and Tasu'a are all voluntary fasts recommended in Islam.Qada' is not voluntary; it is an obligatory fast observed to compensate for missed Ramadan fasts.Thus, Qada' is the exception among the listed options.Answer:B. Qada'